Accept pg_upgraded tuples during multixact freezing

The new MultiXact freezing routines introduced by commit 8e9a16ab
neglected to consider tuples that came from a pg_upgrade'd database; a
vacuum run that tried to freeze such tuples would die with an error such
as
ERROR: MultiXactId 11415437 does no longer exist -- apparent wraparound

To fix, ensure that GetMultiXactIdMembers is allowed to return empty
multis when the infomask bits are right, as is done in other callsites.

Per trouble report from F-Secure.

In passing, fix a copy&paste bug reported by Andrey Karpov from VIVA64
from their PVS-Studio static checked, that instead of setting relminmxid
to Invalid, we were setting relfrozenxid twice.  Not an important
mistake because that code branch is about relations for which we don't
use the frozenxid/minmxid values at all in the first place, but seems to
warrants a fix nonetheless.