pg_dump assertion failure with "-n pg_catalog"

On version 15 or later:

pg_dump -n pg_catalog postgres > /dev/null
pg_dump: pg_dump.c:13291: dumpCollation: Assertion `collcollate !=
((void *)0)' failed.

Patch attached.

I tried adding a test, but dumping pg_catalog seems to only be
supported for research purposes, so it's not clear what the test should
be testing.

If we went almost two release cycles without anyone noticing, then
perhaps we should just get rid of the ability to dump pg_catalog. But I
think the attached patch clarifies things regardless.

--
Jeff Davis
PostgreSQL Contributor Team - AWS