Fix several edge cases in skin editor gameplay scene opening flow

[bdach]

I'm bundling all three fixes together because they're in the same place and small.

• Closes Opening the skin editor while a 0 hitobject beatmap is selected crashes to desktop #25634
  • When the skin editor is opened with a no-object beatmap globally selected, the "Beatmap has no hitobjects!" notification will display, the player screen will hang around for 3 seconds, and then it will exit itself. See 9d39b70.
• Closes System.NullReferenceException: Object reference not set to an instance of an object. #25664
  • This presents when opening the gameplay scene when the dummy beatmap is active. The fix above partially addresses this; the other things that address this are 055fb5b (it seems generally wrong to be changing the player's activity to "watching replay" in a skin editor flow of all places), and 063694f (if the dummy beatmap is active, do not attempt to progress to gameplay at all. Cheap cop-out, you could probably try harder to find something to play, but honestly I'd rather not.)
• Closes System.InvalidCastException: Unable to cast object of type 'osu.Game.Rulesets.Taiko.Beatmaps.TaikoBeatmap' to type 'osu.Game.R... #25663
  • Presents when going to gameplay from main menu, when ambient game ruleset is different from the ruleset of the current ambient game beatmap. Fixed by 8754fa4.

All scenarios covered by unit tests. Decisions on what to do in each situations were ad hoc and are generally disputable, I ended up going the easiest route in all cases basically.

An alternative to this that would fix the first two issues could be to revert #25568, if one was to go this way I suppose. The third was present even before that PR.