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 <aschultz@redhat.com>
(cherry picked from commit faec3e0801)

tripleo_common/image/image_uploader.py

@@ -2117,6 +2117,9 @@ class PythonImageUploader(BaseImageUploader):
         digest = image['digest']
         created = image['created']
         labels = config['config'].get('Labels', {})
+        # NOTE: labels can be null
+        if labels is None:
+            labels = {}
         architecture = config['architecture']
         image_os = config['os']
         return {