[SPARK-20304][SQL] AssertNotNull should not include path in string representation

## What changes were proposed in this pull request?
AssertNotNull's toString/simpleString dumps the entire walkedTypePath. walkedTypePath is used for error message reporting and shouldn't be part of the output.

## How was this patch tested?
Manually tested.

Author: Reynold Xin <rxin@databricks.com>

Closes #17616 from rxin/SPARK-20304.

(cherry picked from commit 54085538)
Signed-off-by: Xiao Li <gatorsmile@gmail.com>

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

@@ -936,6 +936,8 @@ case class AssertNotNull(child: Expression, walkedTypePath: Seq[String] = Nil)
   override def foldable: Boolean = false
   override def nullable: Boolean = false
 
+  override def flatArguments: Iterator[Any] = Iterator(child)
+
   private val errMsg = "Null value appeared in non-nullable field:" +
     walkedTypePath.mkString("\n", "\n", "\n") +
     "If the schema is inferred from a Scala tuple/case class, or a Java bean, " +