Handle empty revision table in populateArchiveRevId.php

Handle empty revision table in populateArchiveRevId.php

Running update.php for new wikis without any revisions yet fails when
update.php attempts to run populateArchiveRevId.php. This problem
does not exist if using the web installer or running
maintenance/install.php, since both of these generate a Main Page
revision. Manually generating a MediaWiki database by sourcing
maintenance/tables.sql does not generate any revisions, and thus is
susceptible to this problem.

Bug: T203982

Change-Id: Ifd78c50fb1e11f82340cd83a10fa903b7c5fc1e7