Fix null data handling in ServiceCommand.writeCommand()

    private boolean writeCommand(int cmd, String _data) {

        byte buf[] = new byte[8];

        byte[] data = _data.getBytes();

        byte[] data = (_data == null) ? new byte[0] : _data.getBytes();

        int len = data.length;

        // the length of data

        buf[0] = (byte) (len & 0xff);