`ModelBackedDrawable<T>.CreateDrawable()` is R#-annotated to accept a potentially null model. Apply nullability there too for better reading experience.
BeatmapSetInfo.OnlineInfo
IBeatmapSetOnlineInfo
BeatmapSetInfo
UpdateableBeatmapBackgroundSprite
IBeatmapInfo
Closes #4401. Alternative to #4439.
Co-Authored-By: nyquillerium <nyquill@ppy.sh>