[SPARK-18442][SQL] Fix nullability of WrapOption.

## What changes were proposed in this pull request?

The nullability of `WrapOption` should be `false`.

## How was this patch tested?

Existing tests.

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

Closes #15887 from ueshin/issues/SPARK-18442.

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

@@ -341,7 +341,7 @@ case class WrapOption(child: Expression, optType: DataType)
 
   override def dataType: DataType = ObjectType(classOf[Option[_]])
 
-  override def nullable: Boolean = true
+  override def nullable: Boolean = false
 
   override def inputTypes: Seq[AbstractDataType] = optType :: Nil