pgsql: Fix SIGSEGV in pruning for ScalarArrayOp with constant-null arra

Fix SIGSEGV in pruning for ScalarArrayOp with constant-null array.

Not much to be said here: commit 9fdb675fc should have checked
constisnull, didn't.

Per report from Piotr Włodarczyk. Back-patch to v11 where
bug was introduced.

Discussion: /messages/by-id/CAP-dhMr+vRpwizEYjUjsiZ1vwqpohTm+3Pbdt6Pr7FEgPq9R0Q@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/59d3789294cf6d42325e92486b053f1ee5934eb8

Modified Files
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src/backend/partitioning/partprune.c | 7 ++-
src/test/regress/expected/partition_prune.out | 66 +++++++++++++++++++++++++++
src/test/regress/sql/partition_prune.sql | 10 ++++
3 files changed, 82 insertions(+), 1 deletion(-)