Check if sensu_available_checks is set before using it

This came into play during a ansible-playbook -CD run,
since the command module doesn't have a 'check' mode

tasks/plugins.yml

@@ -29,7 +29,7 @@
       mode: 0755
       owner: "{{ sensu_user_name }}"
       group: "{{ sensu_group_name }}"
-    when: "'{{ item }}' in sensu_available_checks.stdout_lines"
+    when: "sensu_available_checks is defined and '{{ item }}' in sensu_available_checks.stdout_lines"
     with_flattened:
       - "{{ group_names }}"
     notify: restart sensu-client service