pwdgrp.Group: allow gr_passwd to be None

pwdgrp.Group: allow gr_passwd to be None

I haven't looked into this too closely, but it seems that Android's
getgr* fill gr_passwd with a null pointer, mapped to python None, which
is not a bytes.

Since this assert seems to be 100% py3 sanity-checking and bup doesn't
actually use the gr_passwd anywhere, it should be safe to pass the None.

Signed-off-by: Muh Muhten <muh.muhten@gmail.com>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
[rlb@defaultvalue.org: adjust commit message; add origin comment]