plugin XEP-0045: avoir crash if subject is not set when _get_room_joined_args is called.

libervia/backend/plugins/plugin_xep_0045.py

@@ -278,7 +278,10 @@ class XEP_0045(object):
             room.roomJID.userhost(),
             XEP_0045._get_occupants(room),
             room.nick,
-            room.subject,
+            # FIXME: getattr below is a Q&D fix as `subject` may not be set in early call
+            #    of _get_rooms_joined (during `join` call). The whole workflow of this
+            #    plugin should be refactored.
+            getattr(room, "subject", ""),
             [s.name for s in room.statuses],
             profile
             ]