pgsql: Do not return NULL for error cases in satisfies_hash_partition()

Do not return NULL for error cases in satisfies_hash_partition().

Since this function is used as a CHECK constraint condition,
returning NULL is tantamount to returning TRUE, which would have the
effect of letting in a row that doesn't satisfy the hash condition.
Admittedly, the cases for which this is done should be unreachable
in practice, but that doesn't make it any less a bad idea.  It also
seems like a dartboard was used to decide which error cases should
throw errors as opposed to returning NULL.

For the checks for NULL input values, I just switched it to returning
false.  There's some argument that an error would be better; but the
case really should be can't-happen in a generated hash constraint,
so it's likely not worth more code for.

For the parent-relation-open-failure case, it seems like we might
as well let relation_open throw an error, instead of having an
impossible-to-diagnose constraint failure.

Back-patch to v11 where this code came in.

Discussion: https://postgr.es/m/24067.1605134819@sss.pgh.pa.us

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/84e31622882358f61e9d3c16c5b4f3187f504a68

Modified Files
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src/backend/partitioning/partbounds.c   | 15 +++++++--------
src/test/regress/expected/hash_part.out | 10 +++-------
2 files changed, 10 insertions(+), 15 deletions(-)