Trivial Description
With `EmptyHashedRelation` introduced in https://github.com/apache/spark/pull/29389, it inspired me that there's a minor optimization we can apply to broadcast hash join and shuffled hash join if build side hashed relation is empty.
If build side hashed relation is empty (i.e. build side is empty)
1.inner join: we don't need to execute stream side at all, just return an empty iterator - https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/joins/HashJoin.scala#L152
2.semi join: we don't need to execute stream side at all, just return an empty iterator - https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/joins/HashJoin.scala#L227 .
This is not common that build side is empty, but in case it is, we can leverage it to not execute stream side at all for better query CPU/IO performance.
