Ensure docker.pull never uses None value for tag
Ensure docker.pull never uses None value for tag

When tag is None, it causes the docker-py library to hang while pulling
the image.

Signed-off-by: Luiz Carvalho <ce3f14872713a9ca19158c744f8de50732acf24f@redhat.com>

projectatomic/atomic-reactor,DBuildService/atomic-reactor,fr34k8/atomic-reactor,DBuildService/atomic-reactor,projectatomic/atomic-reactor,fr34k8/atomic-reactor