Behaviour of "open" serial transfers

Originally raised as https://github.com/gbdev/pandocs/pull/556#pullrequestreview-2153629353:

> I'm now wondering what value is obtained from attempting a transfer when the other GB(C) has not set its bit 7. Could it depend on the last transferred bit?

This can be expanded to "what happens if the serial port is not connected to another console?"; to the best of my knowledge, bits shifted in are 1s, but this isn't stated clearly.