refactor(chess): 重构象棋程序基础结构,使用Nullable重构核心代码

- 更新了多个文件的代码结构和类型定义，提高了代码的健壮性和可维护性
- 优化了事件处理、棋子管理和移动逻辑，为后续功能扩展打下坚实基础
- 修复了一些潜在的空指针异常问题，提高了程序的稳定性
- 修复部分bug

Scripts/Src/AbstractPiece.cs

@@ -46,10 +46,10 @@ public abstract class AbstractPiece : IPiece {
         set => data = value;
     }
 
-    public event EventHandler<Vector2I> OnPos;
-    public event EventHandler<Vector2I> OnMove;
-    public event EventHandler<bool> OnSelected;
-    public event EventHandler<string> OnName;
+    public event EventHandler<Vector2I>? OnPos;
+    public event EventHandler<Vector2I>? OnMove;
+    public event EventHandler<bool>? OnSelected;
+    public event EventHandler<string>? OnName;
 
     public virtual bool Move(Vector2I pos) {
         if (!CanMove(pos)) {
@@ -66,7 +66,7 @@ public abstract class AbstractPiece : IPiece {
         return $"{Name} at {pos}";
     }
 
-    public AbstractPiece(string name = "", Vector2I pos = null) {
+    public AbstractPiece(string name = "", Vector2I? pos = null) {
         this.name = name;
         this.pos = pos ?? new();
     }