refactor(http): allow body_handler to be NULL

If it is NULL, automatic ignore of the HTTP body processing during parse and generate.

docs/Melon Developer Guide.txt

@@ -1412,7 +1412,7 @@ Their definitions can be found in melon/include/mln_types.h.
      a) mln_http_t *mln_http_init(mln_tcp_conn_t *connection, void *data, mln_http_handler body_handler);
         Initialize a HTTP object.
         'body_handler' is a callback handler. When we call mln_http_parse() or mln_http_generate(), this function
-        will be called to process HTTP body.
+        will be called to process HTTP body if it is not `NULL`.
         'data' is a user data. It will be passed to 'body_handler'.
 
      b) void mln_http_destroy(mln_http_t *http);

docs/book/cn/http.md

@@ -28,7 +28,7 @@ mln_http_t *mln_http_init(mln_tcp_conn_t *connection, void *data, mln_http_handl
 typedef int (*mln_http_handler)(mln_http_t *, mln_chain_t **, mln_chain_t **);
 ```
 
-描述：创建并初始化`mln_http_t`结构。`connection`是TCP结构，内含TCP套接字。`data`为体处理函数的用户自定义数据部分，用于辅助请求或响应体的处理。`body_handler`是体处理函数，该函数会在每次调用`mln_http_parse`或`mln_http_generate`函数时被调用。体处理函数有三个参数，分别为：http结构，用于解析或生成HTTP报文的双向链表的头和尾节点。
+描述：创建并初始化`mln_http_t`结构。`connection`是TCP结构，内含TCP套接字。`data`为体处理函数的用户自定义数据部分，用于辅助请求或响应体的处理。`body_handler`是体处理函数，如果该指针非`NULL`，则该函数会在每次调用`mln_http_parse`或`mln_http_generate`函数时被调用。体处理函数有三个参数，分别为：http结构，用于解析或生成HTTP报文的双向链表的头和尾节点。
 
 返回值：成功则返回`mln_http_t`结构指针，否则返回`NULL`
 

docs/book/en/http.md

@@ -28,7 +28,7 @@ mln_http_t *mln_http_init(mln_tcp_conn_t *connection, void *data, mln_http_handl
 typedef int (*mln_http_handler)(mln_http_t *, mln_chain_t **, mln_chain_t **);
 ```
 
-Description: Create and initialize the `mln_http_t` structure. `connection` is a TCP structure, which contains a TCP socket. `data` is the user-defined data part of the body processing function, which is used to assist the processing of the request or response body. `body_handler` is the body handler function, which is called every time the `mln_http_parse` or `mln_http_generate` function is called. The body processing function has three parameters: http structure, the head and tail nodes of the doubly linked list used to parse or generate HTTP packets.
+Description: Create and initialize the `mln_http_t` structure. `connection` is a TCP structure, which contains a TCP socket. `data` is the user-defined data part of the body processing function, which is used to assist the processing of the request or response body. `body_handler` is the body handler function, which is called every time the `mln_http_parse` or `mln_http_generate` function is called if it is not `NULL`. The body processing function has three parameters: http structure, the head and tail nodes of the doubly linked list used to parse or generate HTTP packets.
 
 Return value: return `mln_http_t` structure pointer if successful, otherwise return `NULL`
 

src/mln_http.c

@@ -133,7 +133,7 @@ MLN_FUNC(, int, mln_http_parse, (mln_http_t *http, mln_chain_t **in), (http, in)
     }
     if (ret == M_HTTP_RET_OK || ret == M_HTTP_RET_ERROR) return ret;
 
-    ret = handler(http, in, NULL);
+    if (handler != NULL) ret = handler(http, in, NULL);
     if (ret == M_HTTP_RET_DONE) {
         mln_http_done_set(http, 0);
     }