Fix current beatmap temporarily becoming empty during ruleset change
Fix current beatmap temporarily becoming empty during ruleset change

When changing the ruleset at song select, there was a scenario where it
would be set to default (empty) for one debounce length where this was
not actually required. This occurs when the currently selected beatmap
set has other difficulties which match the target ruleset, specifically.

peppy/osu,smoogipoo/osu,smoogipoo/osu,UselessToucan/osu,peppy/osu-new,smoogipoo/osu,NeoAdonis/osu,peppy/osu,peppy/osu,NeoAdonis/osu,UselessToucan/osu,ppy/osu,UselessToucan/osu,NeoAdonis/osu,ppy/osu,smoogipooo/osu,ppy/osu