Handle muc avatars

20:19:45 OUT <presence to="some@ejabberd-muc/pep." id="651ef258-7565-4cee-bb00-d8bda9c933c5-3C54B" xml:lang="en"><x xmlns="http://jabber.org/protocol/muc">[..]</x>[..]</presence>
20:19:45 IN  <presence id="1590777861690626049" to="pep@bouah.net/poezio-foo" from="some@ejabberd-muc"><c xmlns="http://jabber.org/protocol/caps" ver="zNpEdUqI5c8S4e5yyws5pP1ScVA=" hash="sha-1" node="http://www.process-one.net/en/ejabberd/" /><x xmlns="http://jabber.org/protocol/muc#user"><item /></x><nick xmlns="http://jabber.org/protocol/nick" /></presence>
20:19:45 IN  <presence to="pep@bouah.net/poezio-foo" from="some@ejabberd-muc/pep." id="651ef258-7565-4cee-bb00-d8bda9c933c5-3C54B" xml:lang="en"><c xmlns="http://jabber.org/protocol/caps" hash="sha-1" ver="3wsWe0x0S68wFb35oczsM/aDzGQ=" node="https://poez.io" /><idle xmlns="urn:xmpp:idle:1" since="2019-02-13T20:19:45.312783+09:00" /><x xmlns="vcard-temp:x:update"><photo>bc59cfde8c748d222d643c1b2171af95ed1c9d6e</photo></x><x xmlns="http://jabber.org/protocol/muc#user"><item jid="pep@bouah.net/poezio-foo" role="moderator" affiliation="owner" /><status code="201" /><status code="110" /></x><nick xmlns="http://jabber.org/protocol/nick" /></presence>
20:19:45 IN  <message type="groupchat" to="pep@bouah.net/poezio-foo" from="some@ejabberd-muc" id="651ef258-7565-4cee-bb00-d8bda9c933c5-3C54D"><subject /></message>

That first presence IN coming from the mucjid, without any nick, is wrongly interpreted by poezio as another user, and /names counts "2" users in this newly created room.

Poezio should ignore it when it comes to user count, at least. Then what we do with the avatar.. I guess that's up to @linkmauve.