ChangeField: skip PatchSets with null revisions

The revision field has notNull = false, so it may be null. Current
usages always call setRevision() on new PatchSets, but old changes in
the DB may be corrupt. Avoid an NPE when indexing these old changes.

Change-Id: I7467e94bd1b5d1b77e29dd26925463152160512d
(cherry picked from commit 849c5c8e00ed390af667a42fe387872e416c781a)