Add fallback for cases where `BeatmapSet` may be null
Add fallback for cases where `BeatmapSet` may be null

Seems to only happen in tests, but better safe than sorry.

peppy/osu,peppy/osu,NeoAdonis/osu,NeoAdonis/osu,ppy/osu,NeoAdonis/osu,ppy/osu,ppy/osu,peppy/osu,smoogipoo/osu,smoogipoo/osu,smoogipooo/osu,smoogipoo/osu