fix: suppress InspectCode warnings in KiaiHitExplosion - make skinnable properly nullable

Agent-Logs-Url: https://github.com/winnerspiros/osu/sessions/f25219ac-7425-4cf9-b931-f7cf70266b1d

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: Copilot

osu.Game.Rulesets.Taiko/UI/KiaiHitExplosion.cs

@@ -21,11 +21,11 @@ public partial class KiaiHitExplosion : Container
 
         private readonly HitType hitType;
 
-        private SkinnableDrawable skinnable = null!;
+        private SkinnableDrawable? skinnable;
 
-        public override double LifetimeStart => skinnable?.Drawable?.LifetimeStart ?? base.LifetimeStart;
+        public override double LifetimeStart => skinnable?.Drawable.LifetimeStart ?? base.LifetimeStart;
 
-        public override double LifetimeEnd => skinnable?.Drawable?.LifetimeEnd ?? base.LifetimeEnd;
+        public override double LifetimeEnd => skinnable?.Drawable.LifetimeEnd ?? base.LifetimeEnd;
 
         public KiaiHitExplosion(DrawableHitObject judgedObject, HitType hitType)
         {