vacuumdb: Don't skip empty relations in --missing-stats-only mode.

vacuumdb: Don't skip empty relations in --missing-stats-only mode.

Presently, --missing-stats-only skips relations with reltuples set
to 0 because empty relations don't get optimizer statistics.
However, before v14, a reltuples value of 0 was ambiguous: it could
either mean the relation is empty, or it could mean that it hadn't
yet been vacuumed or analyzed.  (Commit 3d351d916b taught v14 and
newer to use -1 for the latter case.)  This ambiguity can cause
--missing-stats-only to inadvertently skip relations that need
optimizer statistics after upgrades to v18 and newer (since
reltuples is now transferred from the old cluster).

To fix, simply remove the check for reltuples != 0.  This will
cause --missing-stats-only to analyze some empty tables, but that
doesn't seem too terrible a trade-off.

Reported-by: Christoph Berg <myon@debian.org>
Reviewed-by: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/aAjyvW5_fRGNr7yF%40msg.df7cb.de