Currently if GROUPING SETS are empty sets, we will clean up
parse->groupClause and treat it as having no groups. If meanwhile there
are no aggregates in tlist or havingQual, the query would be planned as
if there are no GROUPING SETS, which is not correct.

This patch is just a workaround by petending the query has aggregates
when it finds the GROUPING SETS are empty sets.

Fix issue #11003
Reviewed-by: NAsim R P <pasim@vmware.com>
Reviewed-by: NPaul Guo <pguo@pivotal.io>