plugin XEP-0045: fix early return in availableReceived blocking room join:

On room join, the room object is created before the server's self-presence arrives.
`_getRoomUser()` thus returns `user=None` for the initial presence, which led to
`super().availableReceived(presence)` not being called, resulting in wrong typing of the
room and the room not really being joined.

libervia/backend/plugins/plugin_xep_0045.py

@@ -1500,9 +1500,9 @@ class LiberviaMUCClient(muc.MUCClient):
 
     def availableReceived(self, presence) -> None:
         """Handle available presence."""
-        room, user = self._getRoomUser(presence)
+        room, __ = self._getRoomUser(presence)
 
-        if room is None or user is None:
+        if room is None:
             return
 
         # we want to keep statuses with room so they can be sent to the frontend via the