plugin XEP-0045: Fix use of empty string for nicks:

If empty string is used for nick in `join` (e.g., due to a bridge call), take use part of
the JID. This fix the use of empty resource, which was resulting in rejection by the
server.

libervia/backend/plugins/plugin_xep_0045.py

@@ -907,7 +907,7 @@ class XEP_0045:
         @return: The joined room.
         @raise AlreadyJoined: The room is already joined by this profile.
         """
-        if nick is None:
+        if not nick:
             nick = client.jid.user
         if options is None:
             options = {}