I didn't test, but after skimming through the code I think that if an invalid user name/group name is provided, os.chown is passed None, which will fail with ValueError.
It would be better to raise an explicit error like "no such user" / "no such group".
That's something you could add to the tests.

Also, you could initialize _user to -1 instead of None, and replace
    if user is None:
        _user = -1
    else:
        [...]
with
    if user is not None:
        [...]

Same goes for _group.