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).
With recent changes, the pathway between refetching (on request) and
refetching (on requirement due to unpopulated files) was combined.
Unfortunately this pathway also added a forced invalidation, which
should not have been applied to the second case.
Closes https://github.com/ppy/osu/issues/19365.
Really this shouldn't happen but I managed to make it happen. Until this
comes up again in a way that matters, let's just fix the LINQ crash from
`SingleOrDefault`.
I've tested this to work as expected in the broken scenario.