azure: don't try to print deployment failure message when there isn't one

playbooks/azure/openshift-cluster/launch.yml

@@ -107,6 +107,7 @@
 
     - debug:
         msg: "{{ (message.stdout | from_json).error.details[0].message }}"
+      when: message.stdout != ""
 
     - assert:
         that: "{{ not deploy.failed }}"