fix: remove nullable annotation from Slider cache field

Agent-Logs-Url: https://github.com/winnerspiros/osu/sessions/bb854639-73b2-45df-98c8-ed1fa6c53a92

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

Author: Copilot

osu.Game.Rulesets.Osu/Objects/Slider.cs

@@ -37,7 +37,7 @@ public double Duration
         public override IList<HitSampleInfo> AuxiliarySamples => cachedAuxiliarySamples ??= CreateSlidingSamples().Concat(TailSamples).ToArray();
 
         // Cached after ApplyDefaults populates TailSamples — stable for the lifetime of this slider instance.
-        private IList<HitSampleInfo>? cachedAuxiliarySamples;
+        private IList<HitSampleInfo> cachedAuxiliarySamples;
 
         private readonly Cached<Vector2> endPositionCache = new Cached<Vector2>();