I have more thoughts on this warning in general (which will likely see
it removed or moved in the future) but this seems like a quick QOL fix
for now.
As mentioned in https://github.com/ppy/osu/discussions/22861.
It is impossible for the callback passed to `ApplyToBackground()` to
receive a null reference. See `OsuScreen.ApplyToBackground()` - if the
background to call the callback on were `null`, then an
`InvalidOperationException` would be thrown instead.
This always looked weird, and from the looks of the structure was
probably unintentional from the start (it is in the logo tracking
container for whatever reason).
I've also changed the cutoffs to 5% rather than zero, as this seems like
a saner method of showing this dialog. With levels 5% or less, the game
is basically inaudible.
Arguably, the cutoff can be increased to 10%.