Azure: use empty dict if input image has no tags

Root images usually have only `valid: true` tag, which is ignored when setting
tags for the new image, thus when combining old tags with new, the variable
containing old tags would be undefined and the ansible task fails.

playbooks/azure/openshift-cluster/tasks/create_image_from_vm.yml

@@ -43,7 +43,7 @@
 
 - name: calculate final tags
   set_fact:
-    final_tags: "{{ input_image_tags_no_valid | combine(image_tags) }}"
+    final_tags: "{{ input_image_tags_no_valid | default({}) | combine(image_tags) }}"
 
 - name: tag image
   command: >