Allow changing nickname on first connect if nick already in use
If I send <presence to="#room@biboumi/nick"><x muc> and nick is already taken on the network (tested using libera.chat) then I do get the <presence from="#room@biboumi/nick" type="error"><error><conflict> that I expect. However, if I immediately respond with a <presence to="#room@biboumi/nick_"><x muc> then instead of being joined with the alt nick, as I would expect, instead I get no reply and the remote disconnects me after some time.
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