All diff checks fail to run when difficulty is named `[ ]`

[STCapricorn]

...and as a result it shows as all green even though no checks happened in the first place.

note that the set-wide checks still run as usual, but they may also be broken because of the diff name as well

[Naxesss]

I suppose this happens because it matches the section regex (e.g. [Metadata] / [HitObjects]) in .osu files.

Will investigate once I get around to it.