[SPARK-27514][SQL] Skip collapsing windows with empty window expressions (apache#538)

## Upstream SPARK-XXXXX ticket and PR link (if not applicable, explain)

github: apache#24411
jira: https://issues.apache.org/jira/browse/SPARK-27514

## What changes were proposed in this pull request?

A previous change moved the removal of empty window expressions to the RemoveNoopOperations rule, which comes after the CollapseWindow rule. Therefore, by the time we get to CollapseWindow, we aren't guaranteed that empty windows have been removed. This change checks that the window expressions are not empty, and only collapses the windows if both windows are non-empty.

A lengthier description and repro steps here: https://issues.apache.org/jira/browse/SPARK-27514

## How was this patch tested?

A unit test, plus I reran the breaking case mentioned in the Jira ticket.

Author: yifeih

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -798,6 +798,7 @@ object CollapseWindow extends Rule[LogicalPlan] {
   def apply(plan: LogicalPlan): LogicalPlan = plan transformUp {
     case w1 @ Window(we1, ps1, os1, w2 @ Window(we2, ps2, os2, grandChild))
         if ps1 == ps2 && os1 == os2 && w1.references.intersect(w2.windowOutputSet).isEmpty &&
+          we1.nonEmpty && we2.nonEmpty &&
           // This assumes Window contains the same type of window expressions. This is ensured
           // by ExtractWindowFunctions.
           WindowFunctionType.functionType(we1.head) == WindowFunctionType.functionType(we2.head) =>

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/CollapseWindowSuite.scala

@@ -89,4 +89,15 @@ class CollapseWindowSuite extends PlanTest {
     val optimized = Optimize.execute(query.analyze)
     comparePlans(optimized, expected)
   }
+
+  test("Skip windows with empty window expressions") {
+    val query = testRelation
+      .window(Seq(), partitionSpec1, orderSpec1)
+      .window(Seq(sum(a).as('sum_a)), partitionSpec1, orderSpec1)
+
+    val optimized = Optimize.execute(query.analyze)
+    val correctAnswer = query.analyze
+
+    comparePlans(optimized, correctAnswer)
+  }
 }