Add support for transformation of optional children.

Author: marmbrus

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala

@@ -187,6 +187,14 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] {
         } else {
           arg
         }
+      case Some(arg: TreeNode[_]) if children contains arg =>
+        val newChild = arg.asInstanceOf[BaseType].transformDown(rule)
+        if (!(newChild fastEquals arg)) {
+          changed = true
+          Some(newChild)
+        } else {
+          Some(arg)
+        }
       case m: Map[_,_] => m
       case args: Traversable[_] => args.map {
         case arg: TreeNode[_] if children contains arg =>
@@ -231,6 +239,14 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] {
         } else {
           arg
         }
+      case Some(arg: TreeNode[_]) if children contains arg =>
+        val newChild = arg.asInstanceOf[BaseType].transformUp(rule)
+        if (!(newChild fastEquals arg)) {
+          changed = true
+          Some(newChild)
+        } else {
+          Some(arg)
+        }
       case m: Map[_,_] => m
       case args: Traversable[_] => args.map {
         case arg: TreeNode[_] if children contains arg =>