Allow null users to still be displayed in the participant list

The fix here is correcting the access of `user.Country`. The deicision
to have null users display is because this is the best we can do (if
osu-web could not resolve the user). We still want the users in the
lobby to be aware of this user's presence, rather than hiding them from
view.

osu-stable does a similar thing, showing these users as `[Loading]`. I
decided to go with blank names instead because having *any* text there
causes confusion. We can iterate on this in future design updates.

osu.Game/Tests/Visual/Multiplayer/TestMultiplayerClient.cs

@@ -28,6 +28,8 @@ namespace osu.Game.Tests.Visual.Multiplayer
 
         public void AddUser(User user) => ((IMultiplayerClient)this).UserJoined(new MultiplayerRoomUser(user.Id) { User = user });
 
+        public void AddNullUser(int userId) => ((IMultiplayerClient)this).UserJoined(new MultiplayerRoomUser(userId));
+
         public void RemoveUser(User user)
         {
             Debug.Assert(Room != null);