[SPARK-24788][SQL] RelationalGroupedDataset.toString with unresolved exprs should not fail

[maropu]

What changes were proposed in this pull request?

In the current master, toString throws an exception when RelationalGroupedDataset has unresolved expressions;

scala> spark.range(0, 10).groupBy("id")
res4: org.apache.spark.sql.RelationalGroupedDataset = RelationalGroupedDataset: [grouping expressions: [id: bigint], value: [id: bigint], type: GroupBy]

scala> spark.range(0, 10).groupBy('id)
org.apache.spark.sql.catalyst.analysis.UnresolvedException: Invalid call to dataType on unresolved object, tree: 'id
  at org.apache.spark.sql.catalyst.analysis.UnresolvedAttribute.dataType(unresolved.scala:105)
  at org.apache.spark.sql.RelationalGroupedDataset$$anonfun$12.apply(RelationalGroupedDataset.scala:474)
  at org.apache.spark.sql.RelationalGroupedDataset$$anonfun$12.apply(RelationalGroupedDataset.scala:473)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
  at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)
  at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
  at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
  at scala.collection.AbstractTraversable.map(Traversable.scala:104)
  at org.apache.spark.sql.RelationalGroupedDataset.toString(RelationalGroupedDataset.scala:473)
  at scala.runtime.ScalaRunTime$.scala$runtime$ScalaRunTime$$inner$1(ScalaRunTime.scala:332)
  at scala.runtime.ScalaRunTime$.stringOf(ScalaRunTime.scala:337)
  at scala.runtime.ScalaRunTime$.replStringOf(ScalaRunTime.scala:345)

This pr fixed code to handle the unresolved case in RelationalGroupedDataset.toString.

Closes #21752

How was this patch tested?

Added tests in DataFrameAggregateSuite.

[maropu]

@gatorsmile KeyValueGroupedDataset has the same issue? It seems there is no chance for KeyValueGroupedDataset to have unresolved exprs.
#21752 (comment)

[SparkQA]

Test build #93962 has finished for PR 21964 at commit c4e7490.

• This patch fails due to an unknown error code, -9.
• This patch merges cleanly.
• This patch adds no public classes.

[maropu]

retest this please

[SparkQA]

Test build #93975 has finished for PR 21964 at commit c4e7490.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[maropu]

retest this please

[SparkQA]

Test build #93995 has finished for PR 21964 at commit c4e7490.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[maropu]

retest this please

[SparkQA]

Test build #94007 has finished for PR 21964 at commit c4e7490.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[maropu]

@gatorsmile ping

[gatorsmile]

LGTM

Thanks! Merged to master.