image_uploader/base handle when an image has null labels

We missed to patch the _inspect method in BaseUploader when we did:
I67ca5956a44c3ee908e21dcf3b476e2c6c5c7f9e

This patch addresses that.

Change-Id: I7986c585160af2534365d8aff0a3e0d95cd356f4
Related-Bug: #1857012
(cherry picked from commit 6c8979720a)

tripleo_common/image/image_uploader.py

@@ -657,6 +657,9 @@ class BaseImageUploader(object):
         created = config['created']
         docker_version = config.get('docker_version', '')
         labels = config['config'].get('Labels', {})
+        # NOTE: labels can be null
+        if labels is None:
+            labels = {}
         architecture = config['architecture']
         image_os = config['os']