Fix nullability of map values of ExternalMapToCatalyst.

Author: ueshin

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

@@ -601,7 +601,8 @@ case class ExternalMapToCatalyst private(
 
   override def foldable: Boolean = false
 
-  override def dataType: MapType = MapType(keyConverter.dataType, valueConverter.dataType)
+  override def dataType: MapType = MapType(
+    keyConverter.dataType, valueConverter.dataType, valueContainsNull = valueConverter.nullable)
 
   override def eval(input: InternalRow): Any =
     throw new UnsupportedOperationException("Only code-generated evaluation is supported")