Intended to be a temporary fix for
https://github.com/ppy/osu/issues/17713 while a more permanent solution
is established. A proper fix is actually quite simple, but updating the
test to work with it is... *slightly* more complicated.
This hotfix will mean that if a `UserJoined` event arrives during the
unobserved window during room join (of the local user), the local user
still won't see the joining user, but the game won't crash as a result.
This was an implicit requirement until now, but not well documented
everywhere. Adding this makes it much easier to understand the
requirement (and probably safer).