image_uploader: handle when an image has null labels
image_uploader: handle when an image has null labels

If an image inspect returns empty Labels, set the labels to an empty
dictionary.

Change-Id: I67ca5956a44c3ee908e21dcf3b476e2c6c5c7f9e
Closes-Bug: #1857012
Co-Authored-By: Alex Schultz <6963656cd5f1036a86f38d6c01e67c3f63074c58@redhat.com>

openstack/tripleo-common,openstack/tripleo-common