API: Fix list=deletedrevs with pre-1.5 revisions

In pre-1.5, deleted revisions have the text in the ar_text field of the
archive table with empty ar_text_id. Adjust the JOIN used for
drprop=content to take this situation into account, and the logic for
extracting the revision content to match.

Bug: 67699
Change-Id: I3672435ac97fa1eff2bdf80b24c470525c93ab27
Merge "API: Fix list=deletedrevs with pre-1.5 revisions"