Issue callback for receive timeout in uart-stm32 async with 0 bytes received

[erian747]

After starting a uart async reception with uart_rx_enable i was i bit suprised that there were no call to my async event callback

In this case im making a custom modbus client (as the modbus client used in zephyr doesnt support async)
No slave were connected so a zero byte receive length is expected but i still want to be notified that rx did timeout

So in the code below i think the async callback should be called with either UART_RX_RDY and 0 bytes length
or add a new uart_event_type UART_RX_TIMEOUT

is this a good idea, or maybe is have misunderstood how the async api works?