Stop logging Unknown GroupMembership for UUID: null

Some chunk of code in Gerrit is frequently passing a null AccountGroup.UUID
into the membership checker. For most Java collections (which memberships
looks like), .contains(null) is just gracefully false. Honor that behavior
by gracefully skipping a null UUID.

Continue to log any UUIDs which have no registered backends. These may be
errors caused by plugins not loading that an admin should pay attention to
and try to resolve.

Change-Id: Id0a24db6ac57574315cb93c76ad0e0f180d67352
(cherry picked from commit 74861171bc7bdaedd8febc333d082455208c7bcd)
1 file changed