This can happen if the online IDs are not present in the `.osu` files.
Previously this was only working due to the early logic in the import
process (that relies on matching all online IDs perfectly).
Was originally relying on the fact that this would be triggered due to a
null `LastOnlineUpdate`, but wouldn't cover the case of beatmaps with no
`OnlineID`.
As far as I can tell all accesses are safe due to update thread
guarantees. The only weird one may be async writes during a
`BlockAllOperations`, but the `Compact` loop should handle this quite
amicably.