[SPARK-27221][SQL] Improve the assert error message in TreeNode.parseToJson

## What changes were proposed in this pull request?

When `TreeNode.parseToJson` may throw an assert error without any error message when a TreeNode is not implemented properly, and it's hard to find the bad TreeNode implementation.

This PR adds the assert message to improve the error, like what `TreeNode.jsonFields` does.

## How was this patch tested?

Jenkins

Closes apache#24159 from zsxwing/SPARK-27221.

Authored-by: Shixiong Zhu <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: zsxwing

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

@@ -630,7 +630,7 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
     val fieldNames = getConstructorParameterNames(getClass)
     val fieldValues = productIterator.toSeq ++ otherCopyArgs
     assert(fieldNames.length == fieldValues.length, s"${getClass.getSimpleName} fields: " +
-      fieldNames.mkString(", ") + s", values: " + fieldValues.map(_.toString).mkString(", "))
+      fieldNames.mkString(", ") + s", values: " + fieldValues.mkString(", "))
 
     fieldNames.zip(fieldValues).map {
       // If the field value is a child, then use an int to encode it, represents the index of
@@ -683,7 +683,8 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
       try {
         val fieldNames = getConstructorParameterNames(p.getClass)
         val fieldValues = p.productIterator.toSeq
-        assert(fieldNames.length == fieldValues.length)
+        assert(fieldNames.length == fieldValues.length, s"${getClass.getSimpleName} fields: " +
+          fieldNames.mkString(", ") + s", values: " + fieldValues.mkString(", "))
         ("product-class" -> JString(p.getClass.getName)) :: fieldNames.zip(fieldValues).map {
           case (name, value) => name -> parseToJson(value)
         }.toList