Corrected changed_when state for root account lock.

tasks/lock_root.yml

@@ -5,7 +5,7 @@
     name: root
     expires: 1.0
   register: lock_root
-  changed_when: not 'no changes' in lock_root.stderr
+  changed_when: not (lock_root.stderr is defined and 'no changes' in lock_root.stderr)
 
 - name: Disable root SSH login
   lineinfile: