[SPARK-20017][SQL] change the nullability of function 'StringToMap' from 'false' to 'true'

## What changes were proposed in this pull request?

Change the nullability of function `StringToMap` from `false` to `true`.

Author: zhaorongsheng <334362872@qq.com>

Closes #17350 from zhaorongsheng/bug-fix_strToMap_NPE.

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

@@ -390,6 +390,8 @@ case class CreateNamedStructUnsafe(children: Seq[Expression]) extends CreateName
     Examples:
       > SELECT _FUNC_('a:1,b:2,c:3', ',', ':');
        map("a":"1","b":"2","c":"3")
+      > SELECT _FUNC_('a');
+       map("a":null)
   """)
 // scalastyle:on line.size.limit
 case class StringToMap(text: Expression, pairDelim: Expression, keyValueDelim: Expression)
@@ -407,7 +409,7 @@ case class StringToMap(text: Expression, pairDelim: Expression, keyValueDelim: E
 
   override def inputTypes: Seq[AbstractDataType] = Seq(StringType, StringType, StringType)
 
-  override def dataType: DataType = MapType(StringType, StringType, valueContainsNull = false)
+  override def dataType: DataType = MapType(StringType, StringType)
 
   override def checkInputDataTypes(): TypeCheckResult = {
     if (Seq(pairDelim, keyValueDelim).exists(! _.foldable)) {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ComplexTypeSuite.scala

@@ -251,6 +251,9 @@ class ComplexTypeSuite extends SparkFunSuite with ExpressionEvalHelper {
   }
 
   test("StringToMap") {
+    val expectedDataType = MapType(StringType, StringType, valueContainsNull = true)
+    assert(new StringToMap("").dataType === expectedDataType)
+
     val s0 = Literal("a:1,b:2,c:3")
     val m0 = Map("a" -> "1", "b" -> "2", "c" -> "3")
     checkEvaluation(new StringToMap(s0), m0)
@@ -271,6 +274,10 @@ class ComplexTypeSuite extends SparkFunSuite with ExpressionEvalHelper {
     val m4 = Map("a" -> "1", "b" -> "2", "c" -> "3")
     checkEvaluation(new StringToMap(s4, Literal("_")), m4)
 
+    val s5 = Literal("a")
+    val m5 = Map("a" -> null)
+    checkEvaluation(new StringToMap(s5), m5)
+
     // arguments checking
     assert(new StringToMap(Literal("a:1,b:2,c:3")).checkInputDataTypes().isSuccess)
     assert(new StringToMap(Literal(null)).checkInputDataTypes().isFailure)