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/sstream/examples/main.c

@@ -12,7 +12,7 @@ int main(int argc, char **argv) {
         return 1;
     }
     ring = scc_sstream_ref_ring(&stream);
-    Assert(ring != null);
+    Assert(ring != nullptr);
 
     printf("Reading file: %s\n", filename);
 

libs/sstream/include/scc_pos_log.h

@@ -14,8 +14,8 @@ extern logger_t __scc_usr_log;
                          "%s:%zu:%zu: ", (pos).name, (pos).line, (pos).col);   \
         scc_snprintf(_full_msg + _n, sizeof(_full_msg) - _n, fmt,              \
                      ##__VA_ARGS__);                                           \
-        __scc_usr_log.handler(&__scc_usr_log, level, null, 0, null, "%s",      \
-                              _full_msg);                                      \
+        __scc_usr_log.handler(&__scc_usr_log, level, nullptr, 0, nullptr,      \
+                              "%s", _full_msg);                                \
     } while (0)
 
 #define SCC_DEBUG(pos, fmt, ...)                                               \

libs/sstream/src/scc_pos_log.c

@@ -7,7 +7,7 @@ static int user_log_handler(logger_t *module, log_level_t level,
     /* clang-format off */
     (void) module, (void)file, (void)line, (void)func; // 不再使用
 
-    const char *level_str = null;
+    const char *level_str = nullptr;
     switch (level) {
     case LOG_LEVEL_DEBUG: level_str = "DEBUG"; break;
     case LOG_LEVEL_INFO:  level_str = "INFO "; break;

libs/sstream/src/scc_sstream.c

@@ -67,7 +67,7 @@ static int sstream_scan_at(scc_sstream_t *stream, scc_pos_t scan_pos,
 
 // 环形缓冲区填充回调（通过 userdata 获取流对象）
 static cbool fill_func(scc_sstream_char_t *out, void *userdata) {
-    Assert(out != null && userdata != null);
+    Assert(out != nullptr && userdata != nullptr);
     scc_sstream_t *stream = (scc_sstream_t *)userdata;
     if (stream->fill_pos.offset >= stream->len)
         return false; // 已到文件尾
@@ -81,9 +81,9 @@ static cbool fill_func(scc_sstream_char_t *out, void *userdata) {
 }
 
 int scc_sstream_init(scc_sstream_t *stream, const char *fname, int ring_size) {
-    Assert(stream != null && fname != null);
+    Assert(stream != nullptr && fname != nullptr);
     scc_file_t file = scc_fopen(fname, SCC_FILE_READ);
-    if (file == null) {
+    if (file == nullptr) {
         LOG_ERROR("Failed to open file: %s", fname);
         return 1;
     }
@@ -123,13 +123,13 @@ int scc_sstream_init_by_buffer(scc_sstream_t *stream, const char *buffer,
 }
 
 scc_sstream_ring_t *scc_sstream_to_ring(scc_sstream_t *stream) {
-    Assert(stream != null);
+    Assert(stream != nullptr);
     stream->used++;
     return &stream->ring;
 }
 
 void scc_sstream_drop_ring(scc_sstream_ring_t *ring) {
-    Assert(ring != null && ring->userdata != null);
+    Assert(ring != nullptr && ring->userdata != nullptr);
     scc_sstream_t *stream = (scc_sstream_t *)ring->userdata;
     if (stream->used > 0) {
         stream->used--;
@@ -139,14 +139,14 @@ void scc_sstream_drop_ring(scc_sstream_ring_t *ring) {
 }
 
 void scc_sstream_drop(scc_sstream_t *stream) {
-    Assert(stream != null);
+    Assert(stream != nullptr);
     if (stream->used) {
         LOG_FATAL("drop sstream must be drop ring before ref [%d]",
                   stream->used);
     }
     if (stream->src && stream->owned_src) {
         scc_free((void *)stream->src);
-        stream->src = null;
+        stream->src = nullptr;
     }
     scc_ring_free(stream->ring);
 }