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/Lib/RPMessage.cs

@@ -4,20 +4,21 @@ using Godot.Collections;
 namespace RPPackage {
 public class RPMessage
 {
-    public string Type { get; set; }
-    public string Cmd { get; set; }
-    public Dictionary Data { get; set; }
+    public string Type { get; set; } = string.Empty;
+    public string Cmd { get; set; } = string.Empty;
+    public Dictionary? Data { get; set; }
     
-    public string Uid { get; set; }
-    public string Token { get; set; }
-    public string Code { get; set; }
+    public string? Uid { get; set; }
+    public string? Token { get; set; }
+    public string? Code { get; set; }
     public Dictionary ToDictionary()
     {
-        var dict = new Dictionary();
-        if (Type != null)
-            dict.Add("type", Type);
-        if (Cmd != null)
-            dict.Add("cmd", Cmd);
+        var dict = new Dictionary {
+            // if (Type != null)
+            { "type", Type },
+            // if (Cmd != null)
+            { "cmd", Cmd }
+        };
         if (Data != null)
             dict.Add("data", Data);