pgsql: Fix style violations in syscache lookups.

Fix style violations in syscache lookups.

Project style is to check the success of SearchSysCacheN and friends
by applying HeapTupleIsValid to the result. A tiny minority of calls
creatively did it differently. Bring them into line with the rest.

This is just cosmetic, since HeapTupleIsValid is indeed just a null
check at the moment ... but that may not be true forever, and in any
case it puts a mental burden on readers who may wonder why these
call sites are not like the rest.

Back-patch to v11 just to keep the branches in sync. (The bulk of these
errors seem to have originated in v11 or v12, though a few are old.)

Per searching to see if anyplace else had made the same error
repaired in 62148c352.

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/000f557c3111add0e5aa28936ba8ad49ff896667

Modified Files
--------------
src/backend/catalog/pg_publication.c | 2 +-
src/backend/commands/indexcmds.c | 2 +-
src/backend/commands/opclasscmds.c | 4 ++--
src/backend/commands/operatorcmds.c | 2 +-
src/backend/commands/tablecmds.c | 11 +++++------
src/backend/optimizer/util/plancat.c | 2 +-
6 files changed, 11 insertions(+), 12 deletions(-)