Fix master/replica race condition when creating null revisions

Fix master/replica race condition when creating null revisions

RevisionStore::newFromNullRevision() carefully reads the latest revision
from the master, but then forgets to instruct emulateMainSlot_1_29() to
do the same, so the lookup query for rev_text_id in the text table is
done against a replica.

This is not usually a problem for null revisions, but it is a problem
when editing a page and then immediately moving it: this causes a null
revision to be created based on a freshly created revision, which
doesn't yet exist on the replica.

Bug: T184670
Change-Id: Ied4db5ee770716ff3edb918d2be722934c79d0c7