pgsql: Fix STRICT check for strict aggregates with NULL ORDER BYcolumn

Fix STRICT check for strict aggregates with NULL ORDER BY columns.

I (Andres) broke this unintentionally in 69c3936a14, by checking
strictness for all input expressions computed for an aggregate, rather
than just the input for the aggregate transition function.

Reported-By: Ondřej Bouda
Bisected-By: Tom Lane
Diagnosed-By: Andrew Gierth
Discussion: https://postgr.es/m/2a505161-2727-2473-7c46-591ed108ac52@email.cz
Backpatch: 11-, like 69c3936a14

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/fd59b29c873915064f7af6a33c968c97e0d59166

Modified Files
--------------
src/backend/executor/execExpr.c          |  2 +-
src/test/regress/expected/aggregates.out | 15 +++++++++++++++
src/test/regress/sql/aggregates.sql      |  7 +++++++
3 files changed, 23 insertions(+), 1 deletion(-)