Use virtual property instead of a field for optional pieces

osu.Game.Rulesets.Catch/Skinning/Default/BananaPiece.cs

@@ -7,6 +7,8 @@ namespace osu.Game.Rulesets.Catch.Skinning.Default
 {
     public class BananaPiece : CatchHitObjectPiece
     {
+        protected override BorderPiece BorderPiece { get; }
+
         public BananaPiece()
         {
             RelativeSizeAxes = Axes.Both;

osu.Game.Rulesets.Catch/Skinning/Default/CatchHitObjectPiece.cs

@@ -21,11 +21,17 @@ public abstract class CatchHitObjectPiece : CompositeDrawable
         [CanBeNull]
         protected DrawableHitObject DrawableHitObject { get; private set; }
 
+        /// <summary>
+        /// A part of this piece that will be faded out while falling in the playfield.
+        /// </summary>
         [CanBeNull]
-        protected BorderPiece BorderPiece;
+        protected virtual BorderPiece BorderPiece => null;
 
+        /// <summary>
+        /// A part of this piece that will be only visible when <see cref="HyperDash"/> is true.
+        /// </summary>
         [CanBeNull]
-        protected HyperBorderPiece HyperBorderPiece;
+        protected virtual HyperBorderPiece HyperBorderPiece => null;
 
         protected override void LoadComplete()
         {

osu.Game.Rulesets.Catch/Skinning/Default/DropletPiece.cs

@@ -9,6 +9,8 @@ namespace osu.Game.Rulesets.Catch.Skinning.Default
 {
     public class DropletPiece : CatchHitObjectPiece
     {
+        protected override HyperBorderPiece HyperBorderPiece { get; }
+
         public DropletPiece()
         {
             Size = new Vector2(CatchHitObject.OBJECT_RADIUS / 2);

osu.Game.Rulesets.Catch/Skinning/Default/FruitPiece.cs

@@ -16,6 +16,9 @@ internal class FruitPiece : CatchHitObjectPiece
 
         public readonly Bindable<FruitVisualRepresentation> VisualRepresentation = new Bindable<FruitVisualRepresentation>();
 
+        protected override BorderPiece BorderPiece { get; }
+        protected override HyperBorderPiece HyperBorderPiece { get; }
+
         public FruitPiece()
         {
             RelativeSizeAxes = Axes.Both;