message: Update websocket_server::pack, allow to pack empty data.
diff: @@ -462,11 +462,6 @@ static PHP_METHOD(swoole_websocket_server, pack)
  RETURN_FALSE;
  }
  - if (length <= 0)
  - {
  - swoole_php_fatal_error(E_WARNING, "data is empty.");
  - }
  -
  if (swoole_http_buffer == NULL)
  {
  swoole_http_buffer = swString_new(SW_HTTP_RESPONSE_INIT_SIZE);
  