Fix pylint warnings inconsistent-return-statements
    
    Add consistent return to all functions and methods that are covered by
    tox -e pylint[23]. I haven't checked if return None is always a good
    idea or if we should rather raise an error.
    
    See: https://pagure.io/freeipa/issue/7326
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>