https: add HTTP 304 as empty response code

modules/access/http/message.c

@@ -678,10 +678,11 @@ unsigned vlc_http_msg_get_retry_after(const struct vlc_http_msg *m)
 }
 
 uintmax_t vlc_http_msg_get_size(const struct vlc_http_msg *m)
-{
+{   /* IETF RFC7230 §3.3.3 */
     if ((m->status / 100) == 1 /* Informational 1xx (implicitly void) */
      || m->status == 204 /* No Content (implicitly void) */
-     || m->status == 205 /* Reset Content (must be explicitly void) */)
+     || m->status == 205 /* Reset Content (must be explicitly void) */
+     || m->status == 304 /* Not Modified */)
         return 0;
 
     const char *str = vlc_http_msg_get_header(m, "Content-Length");