fix creds sync from yaml
fix creds sync from yaml

    Pass down the creds from yaml file only if the values are not empty.
    Enables to use credentials from other authfiles alternatively.
Signed-off-by: Qi Wang <bcc9c0f3bdae19ba83bcc28773714626caad0902@redhat.com>

mtrmac/skopeo,mtrmac/skopeo,mtrmac/skopeo