请教一个问题，QBytearray若是作为函数返回值的时候，
若是读取的是空，他实际返回的是啥，是0，还是null
QBytearray rx_data = now_serial.readAll();//读取串口数据。
QByteArray QIODevice::readAll()
Reads all remaining data from the device, and returns it as a byte array.
This function has no way of reporting errors; returning an empty QByteArray can mean either that no data was curr ently available for reading, or that an error occurred.
