Support UserMerge into anonymous user
Support UserMerge into anonymous user

When UserMerge is used to merge an account into anonymous user,
we shouldn't attempt to operate on MWEchoNotifUser for $newUser,
as it would cause an exception.

Change-Id: I726e6115d2c30e1579dae55e57cc800aa61d9d7c

wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo