Fix creation of partition descriptor during concurrent detach+drop

If a partition undergoes DETACH CONCURRENTLY immediately followed by
DROP, this could cause a problem for a concurrent transaction
recomputing the partition descriptor when running a prepared statement,
because it tries to dereference a pointer to a tuple that's not found in
a catalog scan.

The existing retry logic added in commit dbca3469ebf8 is sufficient to
cope with the overall problem, provided we don't try to dereference a
non-existant heap tuple.

Arguably, the code in RelationBuildPartitionDesc() has been wrong all
along, since no check was added in commit 898e5e3290a7 against receiving
a NULL tuple from the catalog scan; that bug has only become
user-visible with DETACH CONCURRENTLY which was added in branch 14.
Therefore, even though there's no known mechanism to cause a crash
because of this, backpatch the addition of such a check to all supported
branches.  In branches prior to 14, this would cause the code to fail
with a "missing relpartbound for relation XYZ" error instead of
crashing; that's okay, because there are no reports of such behavior
anyway.

Author: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/18559-b48286d2eacd9a4e@postgresql.org

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