pgsql: Avoid allocating unnecessary zero-sized array.

Avoid allocating unnecessary zero-sized array.

If there are no aggregates, there is no need to allocate an array of
zero AggStatePerGroupData elements.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3649133b147638f25933c93e3d4f6b920da50332

Modified Files
--------------
src/backend/executor/nodeAgg.c | 29 ++++++++++++++++++++++-------
1 file changed, 22 insertions(+), 7 deletions(-)