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/ChineseChess/CCMain.cs

@@ -33,12 +33,19 @@ public class ChessCore {
         playerSelf = new(board, Player.PlayerType.Human);
         playerOpponent = new(board, Player.PlayerType.Human);
 
-        void func(object _self, IBoard.MoveEventArgs record) {
+        void func(object? _self, IBoard.MoveEventArgs record) {
             // 防止 Undo 时 Selected Clear 出现 Null Pointer Exception
             playerSelf.SelectedClear();
             playerOpponent.SelectedClear();
-            board.AddRecord(board.GetPiece(record.From), board.GetPiece(record.To),
-                record.From, record.To);
+
+            // TODO FIXME it can be simple
+            if (record.From is null || record.To is null) {
+                return;
+            }
+
+            IPiece? from = record.From is not null ? board.GetPiece(record.From) : null;
+            IPiece? to = record.To is not null ? board.GetPiece(record.To) : null; 
+            board.AddRecord(from, to, record.From ?? new(), record.To ?? new());
         }
         playerSelf.OnMove += func;
         playerOpponent.OnMove += func;