MUC: Force muc user real JIDs to bare

Its unspecified if servers need to send a full JID or bare JID here.
Further its also unspecified if they need to send all full JIDs of
all connected resources.

So we implement on what we can actually depend, there needs to be
at least a bare JID.
2 jobs for master in 38 seconds (queued for 10 seconds)
Status Name Job ID Coverage
  Test
failed run-test #24600

00:00:38

 
  Build
skipped run-build #24601
 
Name Stage Failure
failed
run-test Test
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ERROR: Job failed: exit code 1