Update websocket_server::pack, allow to pack empty data.
Update websocket_server::pack, allow to pack empty data.

swoole/swoole-src,tutanhamon/swoole-src,tutanhamon/swoole-src,LinkedDestiny/swoole-src,tutanhamon/swoole-src,LinkedDestiny/swoole-src,swoole/swoole-src,swoole/swoole-src,tutanhamon/swoole-src,LinkedDestiny/swoole-src,tutanhamon/swoole-src,swoole/swoole-src,tutanhamon/swoole-src,LinkedDestiny/swoole-src,swoole/swoole-src,tutanhamon/swoole-src,LinkedDestiny/swoole-src,LinkedDestiny/swoole-src,swoole/swoole-src,swoole/swoole-src,LinkedDestiny/swoole-src