Make BeatmapSetOverlay accept nulls everywhere #2416
A step forward in usability of
Helps with making #2350 tidy.
Reviewing one class or class group at a time would make this so much simpler.
The avatar change is grouped into all of this, changes to how things are displayed in SuccessRate are grouped into all of this, changes to display and interaction of things like Beatmap in ScoresContainer are grouped into all of this.
None of these components have been tested individually, so there's a lot of cases I have to go through to validate the code visually.