[SPARK-21304][SQL] remove unnecessary isNull variable for collection related encoder expressions

## What changes were proposed in this pull request?

For these collection-related encoder expressions, we don't need to create `isNull` variable if the loop element is not nullable.

## How was this patch tested?

existing tests.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #18529 from cloud-fan/minor.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala

@@ -335,7 +335,7 @@ object ScalaReflection extends ScalaReflection {
         // TODO: add walked type path for map
         val TypeRef(_, _, Seq(keyType, valueType)) = t
 
-        CollectObjectsToMap(
+        CatalystToExternalMap(
           p => deserializerFor(keyType, Some(p), walkedTypePath),
           p => deserializerFor(valueType, Some(p), walkedTypePath),
           getPath,

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/objects/objects.scala

@@ -465,7 +465,11 @@ object MapObjects {
       customCollectionCls: Option[Class[_]] = None): MapObjects = {
     val id = curId.getAndIncrement()
     val loopValue = s"MapObjects_loopValue$id"
-    val loopIsNull = s"MapObjects_loopIsNull$id"
+    val loopIsNull = if (elementNullable) {
+      s"MapObjects_loopIsNull$id"
+    } else {
+      "false"
+    }
     val loopVar = LambdaVariable(loopValue, loopIsNull, elementType, elementNullable)
     MapObjects(
       loopValue, loopIsNull, elementType, function(loopVar), inputData, customCollectionCls)
@@ -517,7 +521,6 @@ case class MapObjects private(
 
   override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     val elementJavaType = ctx.javaType(loopVarDataType)
-    ctx.addMutableState("boolean", loopIsNull, "")
     ctx.addMutableState(elementJavaType, loopValue, "")
     val genInputData = inputData.genCode(ctx)
     val genFunction = lambdaFunction.genCode(ctx)
@@ -588,12 +591,14 @@ case class MapObjects private(
       case _ => genFunction.value
     }
 
-    val loopNullCheck = inputDataType match {
-      case _: ArrayType => s"$loopIsNull = ${genInputData.value}.isNullAt($loopIndex);"
-      // The element of primitive array will never be null.
-      case ObjectType(cls) if cls.isArray && cls.getComponentType.isPrimitive =>
-        s"$loopIsNull = false"
-      case _ => s"$loopIsNull = $loopValue == null;"
+    val loopNullCheck = if (loopIsNull != "false") {
+      ctx.addMutableState("boolean", loopIsNull, "")
+      inputDataType match {
+        case _: ArrayType => s"$loopIsNull = ${genInputData.value}.isNullAt($loopIndex);"
+        case _ => s"$loopIsNull = $loopValue == null;"
+      }
+    } else {
+      ""
     }
 
     val (initCollection, addElement, getResult): (String, String => String, String) =
@@ -667,11 +672,11 @@ case class MapObjects private(
   }
 }
 
-object CollectObjectsToMap {
+object CatalystToExternalMap {
   private val curId = new java.util.concurrent.atomic.AtomicInteger()
 
   /**
-   * Construct an instance of CollectObjectsToMap case class.
+   * Construct an instance of CatalystToExternalMap case class.
    *
    * @param keyFunction The function applied on the key collection elements.
    * @param valueFunction The function applied on the value collection elements.
@@ -682,15 +687,19 @@ object CollectObjectsToMap {
       keyFunction: Expression => Expression,
       valueFunction: Expression => Expression,
       inputData: Expression,
-      collClass: Class[_]): CollectObjectsToMap = {
+      collClass: Class[_]): CatalystToExternalMap = {
     val id = curId.getAndIncrement()
-    val keyLoopValue = s"CollectObjectsToMap_keyLoopValue$id"
+    val keyLoopValue = s"CatalystToExternalMap_keyLoopValue$id"
     val mapType = inputData.dataType.asInstanceOf[MapType]
     val keyLoopVar = LambdaVariable(keyLoopValue, "", mapType.keyType, nullable = false)
-    val valueLoopValue = s"CollectObjectsToMap_valueLoopValue$id"
-    val valueLoopIsNull = s"CollectObjectsToMap_valueLoopIsNull$id"
+    val valueLoopValue = s"CatalystToExternalMap_valueLoopValue$id"
+    val valueLoopIsNull = if (mapType.valueContainsNull) {
+      s"CatalystToExternalMap_valueLoopIsNull$id"
+    } else {
+      "false"
+    }
     val valueLoopVar = LambdaVariable(valueLoopValue, valueLoopIsNull, mapType.valueType)
-    CollectObjectsToMap(
+    CatalystToExternalMap(
       keyLoopValue, keyFunction(keyLoopVar),
       valueLoopValue, valueLoopIsNull, valueFunction(valueLoopVar),
       inputData, collClass)
@@ -716,7 +725,7 @@ object CollectObjectsToMap {
  * @param inputData An expression that when evaluated returns a map object.
  * @param collClass The type of the resulting collection.
  */
-case class CollectObjectsToMap private(
+case class CatalystToExternalMap private(
     keyLoopValue: String,
     keyLambdaFunction: Expression,
     valueLoopValue: String,
@@ -748,7 +757,6 @@ case class CollectObjectsToMap private(
     ctx.addMutableState(keyElementJavaType, keyLoopValue, "")
     val genKeyFunction = keyLambdaFunction.genCode(ctx)
     val valueElementJavaType = ctx.javaType(mapType.valueType)
-    ctx.addMutableState("boolean", valueLoopIsNull, "")
     ctx.addMutableState(valueElementJavaType, valueLoopValue, "")
     val genValueFunction = valueLambdaFunction.genCode(ctx)
     val genInputData = inputData.genCode(ctx)
@@ -781,7 +789,12 @@ case class CollectObjectsToMap private(
     val genKeyFunctionValue = genFunctionValue(keyLambdaFunction, genKeyFunction)
     val genValueFunctionValue = genFunctionValue(valueLambdaFunction, genValueFunction)
 
-    val valueLoopNullCheck = s"$valueLoopIsNull = $valueArray.isNullAt($loopIndex);"
+    val valueLoopNullCheck = if (valueLoopIsNull != "false") {
+      ctx.addMutableState("boolean", valueLoopIsNull, "")
+      s"$valueLoopIsNull = $valueArray.isNullAt($loopIndex);"
+    } else {
+      ""
+    }
 
     val builderClass = classOf[Builder[_, _]].getName
     val constructBuilder = s"""
@@ -847,9 +860,17 @@ object ExternalMapToCatalyst {
       valueNullable: Boolean): ExternalMapToCatalyst = {
     val id = curId.getAndIncrement()
     val keyName = "ExternalMapToCatalyst_key" + id
-    val keyIsNull = "ExternalMapToCatalyst_key_isNull" + id
+    val keyIsNull = if (keyNullable) {
+      "ExternalMapToCatalyst_key_isNull" + id
+    } else {
+      "false"
+    }
     val valueName = "ExternalMapToCatalyst_value" + id
-    val valueIsNull = "ExternalMapToCatalyst_value_isNull" + id
+    val valueIsNull = if (valueNullable) {
+      "ExternalMapToCatalyst_value_isNull" + id
+    } else {
+      "false"
+    }
 
     ExternalMapToCatalyst(
       keyName,
@@ -919,9 +940,7 @@ case class ExternalMapToCatalyst private(
 
     val keyElementJavaType = ctx.javaType(keyType)
     val valueElementJavaType = ctx.javaType(valueType)
-    ctx.addMutableState("boolean", keyIsNull, "")
     ctx.addMutableState(keyElementJavaType, key, "")
-    ctx.addMutableState("boolean", valueIsNull, "")
     ctx.addMutableState(valueElementJavaType, value, "")
 
     val (defineEntries, defineKeyValue) = child.dataType match {
@@ -957,16 +976,18 @@ case class ExternalMapToCatalyst private(
         defineEntries -> defineKeyValue
     }
 
-    val keyNullCheck = if (ctx.isPrimitiveType(keyType)) {
-      s"$keyIsNull = false;"
-    } else {
+    val keyNullCheck = if (keyIsNull != "false") {
+      ctx.addMutableState("boolean", keyIsNull, "")
       s"$keyIsNull = $key == null;"
+    } else {
+      ""
     }
 
-    val valueNullCheck = if (ctx.isPrimitiveType(valueType)) {
-      s"$valueIsNull = false;"
-    } else {
+    val valueNullCheck = if (valueIsNull != "false") {
+      ctx.addMutableState("boolean", valueIsNull, "")
       s"$valueIsNull = $value == null;"
+    } else {
+      ""
     }
 
     val arrayCls = classOf[GenericArrayData].getName