[SPARK-18585][SQL] Use `ev.isNull = "false"` if possible for Janino to have a chance to optimize.

## What changes were proposed in this pull request?

Janino can optimize `true ? a : b` into `a` or `false ? a : b` into `b`, or if/else with literal condition, so we should use literal as `ev.isNull` if possible.

## How was this patch tested?

Existing tests.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #16008 from ueshin/issues/SPARK-18585.

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

@@ -61,7 +61,6 @@ case class CreateArray(children: Seq[Expression]) extends Expression {
     ctx.addMutableState("Object[]", values, s"this.$values = null;")
 
     ev.copy(code = s"""
-      final boolean ${ev.isNull} = false;
       this.$values = new Object[${children.size}];""" +
       ctx.splitExpressions(
         ctx.INPUT_ROW,
@@ -78,7 +77,7 @@ case class CreateArray(children: Seq[Expression]) extends Expression {
       s"""
         final ArrayData ${ev.value} = new $arrayClass($values);
         this.$values = null;
-      """)
+      """, isNull = "false")
   }
 
   override def prettyName: String = "array"
@@ -144,7 +143,6 @@ case class CreateMap(children: Seq[Expression]) extends Expression {
     val keyData = s"new $arrayClass($keyArray)"
     val valueData = s"new $arrayClass($valueArray)"
     ev.copy(code = s"""
-      final boolean ${ev.isNull} = false;
       $keyArray = new Object[${keys.size}];
       $valueArray = new Object[${values.size}];""" +
       ctx.splitExpressions(
@@ -177,7 +175,7 @@ case class CreateMap(children: Seq[Expression]) extends Expression {
         final MapData ${ev.value} = new $mapClass($keyData, $valueData);
         this.$keyArray = null;
         this.$valueArray = null;
-      """)
+      """, isNull = "false")
   }
 
   override def prettyName: String = "map"
@@ -301,7 +299,6 @@ case class CreateNamedStruct(children: Seq[Expression]) extends CreateNamedStruc
     ctx.addMutableState("Object[]", values, s"this.$values = null;")
 
     ev.copy(code = s"""
-      boolean ${ev.isNull} = false;
       $values = new Object[${valExprs.size}];""" +
       ctx.splitExpressions(
         ctx.INPUT_ROW,
@@ -317,7 +314,7 @@ case class CreateNamedStruct(children: Seq[Expression]) extends CreateNamedStruc
       s"""
         final InternalRow ${ev.value} = new $rowClass($values);
         this.$values = null;
-      """)
+      """, isNull = "false")
   }
 
   override def prettyName: String = "named_struct"
@@ -333,7 +330,7 @@ case class CreateNamedStruct(children: Seq[Expression]) extends CreateNamedStruc
 case class CreateNamedStructUnsafe(children: Seq[Expression]) extends CreateNamedStructLike {
   override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     val eval = GenerateUnsafeProjection.createCode(ctx, valExprs)
-    ExprCode(code = eval.code, isNull = eval.isNull, value = eval.value)
+    ExprCode(code = eval.code, isNull = "false", value = eval.value)
   }
 
   override def prettyName: String = "named_struct_unsafe"

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

@@ -206,9 +206,8 @@ case class IsNaN(child: Expression) extends UnaryExpression
       case DoubleType | FloatType =>
         ev.copy(code = s"""
           ${eval.code}
-          boolean ${ev.isNull} = false;
           ${ctx.javaType(dataType)} ${ev.value} = ${ctx.defaultValue(dataType)};
-          ${ev.value} = !${eval.isNull} && Double.isNaN(${eval.value});""")
+          ${ev.value} = !${eval.isNull} && Double.isNaN(${eval.value});""", isNull = "false")
     }
   }
 }
@@ -383,7 +382,6 @@ case class AtLeastNNonNulls(n: Int, children: Seq[Expression]) extends Predicate
     ev.copy(code = s"""
       int $nonnull = 0;
       $code
-      boolean ${ev.isNull} = false;
-      boolean ${ev.value} = $nonnull >= $n;""")
+      boolean ${ev.value} = $nonnull >= $n;""", isNull = "false")
   }
 }