`TokenAuth.get_auth_header` returns invalid data when token is `None`

[Meallia]

When connecting to a registry for the first time, without a bearer token, the following Authorization header is sent : Bearer None. It seems to break some registry ( Jfrog's Artifactory in our case ) and make them reply with and invalid auth header.

I checked and neither docker, cri-o, podman nor flux sends an empty authorization header.

[vsoch]

That's strange - is that documented anywhere?

[Meallia]

it seems to have been caused by #148.

before this refactor, no Authorization header were sent when there was no token. Probably an oversight since it seems to be working on most public container registries.