Fix token basic auth header issue

When requesting a token, the basic auth header is always being set even
if there is no username value. This patch corrects this and does not set
the basic auth header if the username is empty.

Also fixes an issue where pulling all tags from a v2 registry succeeds
when the image does not actually exist on the registry.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Upstream-commit: 8bfdad9a0dee26b45b15f985977769ef37888f8a
Component: engine

components/engine/graph/pull.go

@@ -387,6 +387,9 @@ func (s *TagStore) pullV2Repository(eng *engine.Engine, r *registry.Session, out
 		if err != nil {
 			return err
 		}
+		if len(tags) == 0 {
+			return registry.ErrDoesNotExist
+		}
 		for _, t := range tags {
 			if downloaded, err := s.pullV2Tag(eng, r, out, endpoint, repoInfo, t, sf, parallel, auth); err != nil {
 				return err