Solve bytes return in base.py

[janiversen]

Reminder for myself,

This should be changed to None or an empty ModbusResponse
line 163-165 in base.py

if self.broadcast_enable and not request.slave_id:
resp = b"Broadcast write sent - no response expected"
break
these are the lines in async_execute which cause there to be a bytes return
i declared async_execute as returning ModbusResponse to allow type: ignore to work on the return type. Otherwise mypy would complain a lot about this. I do not know why this code was accepted in the first place