message: suppress null member rejoins again
  reverts
  should fix
diff: @@ -65,8 +65,8 @@ function textForMemberEvent(ev) {
  } else if (!ev.getPrevContent().avatar_url && ev.getContent().avatar_url) {
  return senderName + " set a profile picture";
  } else {
  - // hacky hack for https://github.com/vector-im/vector-web/issues/2020
  - return senderName + " rejoined the room.";
  + // suppress null rejoins
  + return '';
  }
  } else {
  if (!ev.target) console.warn("Join message has no target! -- " + ev.getContent().state_key);
  