Closes#16651.
When a ruleset is not available, the `Find` call would return null. When
a null is passed to the constructor, `BeatmapInfo` would create an "osu"
ruleset, which tries to get stored to realm and fails on duplicate
primary key.
Probably need to add better safeties against this (or change that
constructor...) but this will fix the migration process.
Probably not serious enough to pull the build. This only affects
rulesets like karaoke which have custom beatmaps.
81461be49f