Regressed in 60d9c358b8.
In general an `APIBeatmap`'s `BeatmapSet` need not be present. In the
usage site of `osu.Game.Overlays.BeatmapSet.Info`, `Beatmap` and
`BeatmapSet` were actually two separate bindables. Moving the logic to a
helper, and therefore implicitly moving `BeatmapSet` from tracking said
separate bindable to instead refer to `Beatmap.BeatmapSet` broke this.