Default `DrawableStoryboard` to a completed state to avoid state change on empty storyboards

osu.Game/Storyboards/Drawables/DrawableStoryboard.cs

@@ -25,7 +25,7 @@ public class DrawableStoryboard : Container<DrawableStoryboardLayer>
         /// </summary>
         public IBindable<bool> HasStoryboardEnded => hasStoryboardEnded;
 
-        private readonly BindableBool hasStoryboardEnded = new BindableBool();
+        private readonly BindableBool hasStoryboardEnded = new BindableBool(true);
 
         protected override Container<DrawableStoryboardLayer> Content { get; }