`exec_run` with `demux=True`: output is `None` instead of empty `bytes`

When calling exec_run with demux=True, the documentation states that a pair of bytes is returned.

However, if no standard output (or no standard error) is produced, then the corresponding tuple entry is None instead of an empty bytes (b'').

Minimum working example:

import docker
client = docker.from_env()
container = client.containers.run("alpine:latest", detach=True, tty=True)
container.exec_run(("sh", "-c", "echo out >&1"), demux=True)  # ExecResult(exit_code=0, output=(b'out\n', None))
container.exec_run(("sh", "-c", "echo err >&2"), demux=True)  # ExecResult(exit_code=0, output=(None, b'err\n'))

The expected behaviour is that both tuple entries are always bytes. Alternatively, the possibility of returning None should be documented clearly!

Start with the exec_run entry point and run the minimum working example against alpine:latest with demux=True to reproduce both output cases. Done means the behavior matches the expected pair of bytes, or the documentation clearly explains when a tuple entry is None; add coverage for the no-stdout and no-stderr cases if the existing test suite has a relevant location.