Don't lose partitioned table reltuples=0 after relhassubclass=f.

Don't lose partitioned table reltuples=0 after relhassubclass=f.

ANALYZE sets relhassubclass=f when a partitioned table no longer has
partitions.  An ANALYZE doing that proceeded to apply the inplace update
of pg_class.reltuples to the old pg_class tuple instead of the new
tuple, losing that reltuples=0 change if the ANALYZE committed.
Non-partitioning inheritance trees were unaffected.  Back-patch to v14,
where commit 375aed36ad83f0e021e9bdd3a0034c0c992c66dc introduced
maintenance of partitioned table pg_class.reltuples.

Reported by Alexander Lakhin.

Discussion: https://postgr.es/m/a295b499-dcab-6a99-c06e-01cf60593344@gmail.com