fix dict object has no attribute in 02-backup in check mode

tasks/02-backup.yml

@@ -34,5 +34,5 @@
 
 - name: Backup existing zone files
   shell: "mv {{bind9_zone_dir}}/{{item}} {{bind9_zone_dir}}/{{item}}.bak"
-  with_items: "{{existing_zones_result.stdout_lines}}"
+  with_items: "{{existing_zones_result.stdout_lines | default([])}}"
   when: not bind9_initial_setup