handle Docker registry auth config more gracefully

[tgross]

In #10722 it was noted that if the Docker server has been authenticated to the registry, the Nomad task driver won't be able to use that auth, but may be overriding it the authentication options to the API with an empty struct rather than a nil one. That's not the documented way of doing things so this isn't strictly a bug, but it is an annoying papercut we could improve.