refactor(argparse): 将null替换为nullptr以提高C++兼容性

- 在argparse库中将所有null指针常量替换为nullptr
- 更新头文件和源文件中的指针初始化和比较操作
- 修改测试文件中的相关断言检查
- 更新AST定义文件中的注释说明

libs/parser/src/scc_parser.c

@@ -13,7 +13,7 @@ static scc_ast_type_t *dummy_got_type_callback(void *context,
                                                const char *name) {
     (void)context;
     (void)name;
-    return null;
+    return nullptr;
 }
 
 #define ASSIGN_PTR_OR_DEFAULT(assigned_val, value, default)                    \
@@ -21,10 +21,10 @@ static scc_ast_type_t *dummy_got_type_callback(void *context,
 
 void scc_parser_init(scc_parser_t *parser, scc_lexer_tok_ring_t *tok_ring,
                      scc_sema_callbacks_t *callbacks) {
-    Assert(parser != null && tok_ring != null);
+    Assert(parser != nullptr && tok_ring != nullptr);
     parser->ring = tok_ring;
     parser->errcode = 0;
-    parser->translation_unit = null;
+    parser->translation_unit = nullptr;
     if (callbacks) {
         ASSIGN_PTR_OR_DEFAULT(parser->sema_callbacks.on_decl,
                               callbacks->on_decl, dummy_sema_callback);
@@ -43,7 +43,7 @@ void scc_parser_init(scc_parser_t *parser, scc_lexer_tok_ring_t *tok_ring,
         parser->sema_callbacks.on_expr = dummy_sema_callback;
         parser->sema_callbacks.on_type = dummy_sema_callback;
         parser->sema_callbacks.got_type = dummy_got_type_callback;
-        parser->sema_callbacks.context = null;
+        parser->sema_callbacks.context = nullptr;
     }
 }
 
@@ -55,7 +55,7 @@ scc_ast_translation_unit_t *scc_parse_translation_unit(scc_parser_t *parser) {
     scc_ast_translation_unit_t *unit =
         scc_malloc(sizeof(scc_ast_translation_unit_t));
     if (!unit)
-        return null;
+        return nullptr;
     unit->base.type = SCC_AST_TRANSLATION_UNIT;
     scc_vec_init(unit->declarations);
 
@@ -66,7 +66,7 @@ scc_ast_translation_unit_t *scc_parse_translation_unit(scc_parser_t *parser) {
      */
     while (1) {
         scc_ast_decl_t *decl = scc_parse_declaration(parser);
-        if (decl != null) {
+        if (decl != nullptr) {
             scc_vec_push(unit->declarations, decl);
         } else {
             break;
@@ -77,7 +77,7 @@ scc_ast_translation_unit_t *scc_parse_translation_unit(scc_parser_t *parser) {
             break;
         }
         const scc_lexer_tok_t *tok = scc_parser_peek(parser);
-        if (tok == null || tok->type == SCC_TOK_EOF) {
+        if (tok == nullptr || tok->type == SCC_TOK_EOF) {
             break;
         }
     }
@@ -87,7 +87,7 @@ scc_ast_translation_unit_t *scc_parse_translation_unit(scc_parser_t *parser) {
         SCC_ERROR(scc_parser_got_current_pos(parser), "parser error: %d",
                   parser->errcode);
         scc_free(unit);
-        return null;
+        return nullptr;
     }
 
     Assert(unit->base.type == SCC_AST_TRANSLATION_UNIT);