Fixes #2575.
Also removes unnecessary background texture (the actual storyboard background spec wasn't implemented correctly anyway).
Caused potential stutters for components that relied on this check.
GetStoryboard()
- [ ] Depends on https://github.com/ppy/osu-framework/pull/1035
Also ensures some extra fields inside Beatmap have non-null defaults.
Also fixes circular references when using IJsonSerializable.