Deal with the case when CREATE_SERVICE_WORKER_USERS is false.

In this case, `service_worker_users` still needs to be defined because the when condition is applied for each item in the `{{ service_worker_users }}` list.  So that var always has to resolve.

playbooks/roles/edxapp/tasks/deploy.yml

@@ -425,6 +425,14 @@
   tags:
     - manage
     - manage:db
+    
+- name: build service empty worker users list
+  set_fact:
+    service_worker_users: "{{ [] }}"
+  when: not CREATE_SERVICE_WORKER_USERS
+  tags:
+    - manage
+    - manage:db
 
 - name: create service worker users
   shell: "{{ edxapp_venv_bin }}/python ./manage.py lms --settings={{ edxapp_settings }} --service-variant lms manage_user {{ item.username}} {{ item.email }} --unusable-password {% if item.is_staff %} --staff{% endif %}"