We have other safeties which mean that this is not an issue during
gameplay, but in the new `TestSceneHUDOverlayRulesetLayouts` it became
apparent that allowing this to fallback (via `null` return) could lead
to weirdness.
As time goes on, default skin layouts are getting more and more
complicated because of per-ruleset overrides. This was already sort of
apparent in https://github.com/ppy/osu/pull/31527, and I'm about to make
it worse, so before I do, this is a test scene that is supposed to make
it easier to check all possible combinations at a glance.
I'm not sure why this condition was written this obtusely, but
importantly it was also wrong. It fires when the selection contains
multiple items and only some are removed from it, like when
ctrl-click-unselecting an item from a multiple selection.