pgsql: Fix run-time partition pruning code to handle NULL values proper

Fix run-time partition pruning code to handle NULL values properly.

The previous coding just ignored pruning constraints that compare a
partition key to a null-valued expression. This is silly, since really
what we can do there is conclude that all partitions are rejected: the
pruning operator is known strict so the comparison must always fail.

This also fixes the logic to not ignore constisnull for a Const comparison
value. That's probably an unreachable case, since the planner would
normally have simplified away a strict operator with a constant-null input.
But this code has no business assuming that.

David Rowley, per a gripe from me

Discussion: https://postgr.es/m/26279.1528670981@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/be3d90026a3c17c7e6cc23d52430c37df403d869

Modified Files
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src/backend/partitioning/partprune.c | 48 +++++++++++++++++++--------
src/test/regress/expected/partition_prune.out | 14 ++++++++
src/test/regress/sql/partition_prune.sql | 4 +++
3 files changed, 53 insertions(+), 13 deletions(-)