[SPARK-16703][SQL] Remove extra whitespace in SQL generation for window functions

## What changes were proposed in this pull request?

This PR fixes a minor formatting issue of `WindowSpecDefinition.sql` when no partitioning expressions are present.

Before:

```sql
( ORDER BY `a` ASC ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
```

After:

```sql
(ORDER BY `a` ASC ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
```

## How was this patch tested?

New test case added in `ExpressionSQLBuilderSuite`.

Author: Cheng Lian <lian@databricks.com>

Closes #14334 from liancheng/window-spec-sql-format.

1 files changed, 3 insertions, 3 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala
index c0b453dccf..e35192ca2d 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala
@@ -82,16 +82,16 @@ case class WindowSpecDefinition(
     val partition = if (partitionSpec.isEmpty) {
       ""
     } else {
-      "PARTITION BY " + partitionSpec.map(_.sql).mkString(", ")
+      "PARTITION BY " + partitionSpec.map(_.sql).mkString(", ") + " "
     }
 
     val order = if (orderSpec.isEmpty) {
       ""
     } else {
-      "ORDER BY " + orderSpec.map(_.sql).mkString(", ")
+      "ORDER BY " + orderSpec.map(_.sql).mkString(", ") + " "
     }
 
-    s"($partition $order ${frameSpecification.toString})"
+    s"($partition$order${frameSpecification.toString})"
   }
 }