Don’t process pubsub events from full JIDs

I see a few of those in my logs. It may incorrect behaviour from slidge, I'm not sure.

2024-09-04T11:30:04 (E) nbxmpp.dispatcher          (slidge.im) Handler exception:
Traceback (most recent call last):
  File "/home/nicoco/.venv/gajim-nolocal/lib/python3.11/site-packages/nbxmpp/dispatcher.py", line 525, in _execute_handler_chain
    handler['func'](self._client, stanza, properties)
  File "/home/nicoco/src/perso/xmpp/gajim/gajim/common/modules/util.py", line 66, in func_wrapper
    func(self, _con, _stanza, properties)
  File "/home/nicoco/src/perso/xmpp/gajim/gajim/common/modules/user_avatar.py", line 56, in _avatar_metadata_received
    sha = contact.avatar_sha
          ^^^^^^^^^^^^^^^^^^
AttributeError: 'ResourceContact' object has no attribute 'avatar_sha'