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

runtime/scc_utils/src/hashtable.c

@@ -13,7 +13,7 @@ void scc_hashtable_init(scc_hashtable_t *ht,
     ht->tombstone_count = 0;
     ht->cmp_func = cmp_func;
     ht->hash_func = hash_func;
-    Assert(ht->cmp_func != null && ht->hash_func != null);
+    Assert(ht->cmp_func != nullptr && ht->hash_func != nullptr);
 }
 
 static int next_power_of_two(int n) {
@@ -29,12 +29,12 @@ static int next_power_of_two(int n) {
 static scc_hashtable_entry_t *find_entry(scc_hashtable_t *ht, const void *key,
                                          u32 hash) {
     if (ht->entries.cap == 0)
-        return NULL;
+        return nullptr;
 
     u32 index = hash & (ht->entries.cap - 1); // 容量是2的幂
     u32 probe = 0;
 
-    scc_hashtable_entry_t *tombstone = NULL;
+    scc_hashtable_entry_t *tombstone = nullptr;
 
     while (1) {
         scc_hashtable_entry_t *entry = &scc_vec_at(ht->entries, index);
@@ -56,7 +56,7 @@ static scc_hashtable_entry_t *find_entry(scc_hashtable_t *ht, const void *key,
             break;
     }
     LOG_ERROR("hashset_find: hash table is full");
-    return NULL;
+    return nullptr;
 }
 
 static void adjust_capacity(scc_hashtable_t *ht, usize new_cap) {
@@ -71,7 +71,7 @@ static void adjust_capacity(scc_hashtable_t *ht, usize new_cap) {
     ht->entries.size = new_cap;
     ht->entries.cap = new_cap;
     ht->entries.data =
-        scc_realloc(NULL, new_cap * sizeof(scc_hashtable_entry_t));
+        scc_realloc(nullptr, new_cap * sizeof(scc_hashtable_entry_t));
     scc_memset(ht->entries.data, 0, new_cap * sizeof(scc_hashtable_entry_t));
 
     // rehash the all of the old data
@@ -99,7 +99,7 @@ void *scc_hashtable_set(scc_hashtable_t *ht, const void *key, void *value) {
     u32 hash = ht->hash_func(key);
     scc_hashtable_entry_t *entry = find_entry(ht, key, hash);
 
-    void *old_value = NULL;
+    void *old_value = nullptr;
     if (entry->state == ENTRY_ACTIVE) {
         old_value = entry->value;
     } else {
@@ -117,22 +117,22 @@ void *scc_hashtable_set(scc_hashtable_t *ht, const void *key, void *value) {
 
 void *scc_hashtable_get(scc_hashtable_t *ht, const void *key) {
     if (ht->entries.cap == 0)
-        return NULL;
+        return nullptr;
 
     u32 hash = ht->hash_func(key);
     scc_hashtable_entry_t *entry = find_entry(ht, key, hash);
-    return (entry && entry->state == ENTRY_ACTIVE) ? entry->value : NULL;
+    return (entry && entry->state == ENTRY_ACTIVE) ? entry->value : nullptr;
 }
 
 void *scc_hashtable_del(scc_hashtable_t *ht, const void *key) {
     if (ht->entries.cap == 0)
-        return NULL;
+        return nullptr;
 
     u32 hash = ht->hash_func(key);
     scc_hashtable_entry_t *entry = find_entry(ht, key, hash);
 
-    if (entry == NULL || entry->state != ENTRY_ACTIVE)
-        return NULL;
+    if (entry == nullptr || entry->state != ENTRY_ACTIVE)
+        return nullptr;
 
     void *value = entry->value;
     entry->state = ENTRY_TOMBSTONE;

runtime/scc_utils/src/strpool.c

@@ -15,7 +15,7 @@ const char *scc_strpool_intern(scc_strpool_t *pool, const char *str) {
     char *new_str = scc_malloc(len);
     if (!new_str) {
         LOG_ERROR("strpool: Failed to allocate memory for string");
-        return NULL;
+        return nullptr;
     }
     scc_memcpy(new_str, str, len);