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/sccf/src/sccf_builder.c

@@ -14,11 +14,11 @@ void sccf_builder_init(sccf_builder_t *builder) {
     scc_vec_init(builder->relocs);
     scc_vec_init(builder->symtab);
 
-    builder->entry_symbol_name = null;
+    builder->entry_symbol_name = nullptr;
 
-    ///< Push null
+    ///< Push nullptr
     scc_vec_push(builder->strtab, (char)'\0');
-    ///< Push null
+    ///< Push nullptr
     scc_vec_push(builder->symtab, (sccf_sym_t){0});
 }
 
@@ -69,12 +69,12 @@ void sccf_builder_add_section(sccf_builder_t *builder,
 }
 
 const sccf_t *sccf_builder_to_sccf(sccf_builder_t *builder) {
-    if (builder->entry_symbol_name == null) {
+    if (builder->entry_symbol_name == nullptr) {
         builder->sccf.header.entry_point = 0;
     } else {
         sccf_sym_t *sym =
             sccf_builder_get_symbol_unsafe(builder, builder->entry_symbol_name);
-        if (sym == null || sym->sccf_sect_type != SCCF_SECT_CODE) {
+        if (sym == nullptr || sym->sccf_sect_type != SCCF_SECT_CODE) {
             LOG_ERROR("entry symbol %s not found");
             builder->sccf.header.entry_point = 0;
         } else {
@@ -125,14 +125,14 @@ const sccf_t *sccf_builder_to_sccf(sccf_builder_t *builder) {
 }
 
 void sccf_builder_to_buffer(sccf_builder_t *builder, sccf_buffer_t *buffer) {
-    Assert(builder != null && buffer != null);
+    Assert(builder != nullptr && buffer != nullptr);
     sccf_write(sccf_builder_to_sccf(builder), buffer);
 }
 
 void sccf_builder_to_file(sccf_builder_t *builder, const char *file_path) {
-    Assert(builder != null && file_path != null);
+    Assert(builder != nullptr && file_path != nullptr);
     scc_file_t fp = scc_fopen(file_path, SCC_FILE_WRITE);
-    if (fp == null) {
+    if (fp == nullptr) {
         LOG_ERROR("file can't open %s", file_path);
         return;
     }