Change to an early continue for a NULL IrcUser

Commit e9096505f07fc0c08a7c36f3680c1fde975d4f80 changed the behaviour
in joinIrcUsers() from skipping a NULL IrcUser or an already tracked
IrcUser to updating the usermodes of said IrcUser. The call to
addUserMode() will call isKnownUser() which will log a warning
of 'received IrcUser Nullpointer' and return false. This is safe
from crashing, but shouldn't be allowed to happen.