[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.