check if var is defined

roles/webserver/tasks/main.yml

@@ -7,9 +7,13 @@
       msg: Debug
   # install software
   - import_tasks: install_apache.yml
-    when: apache == true
+    when:
+      - apache is defined
+      - apache == true
   - import_tasks: install_nginx.yml
-    when: nginx == true
+    when:
+      - nginx is defined
+      - nginx == true
   - name: webserver | certbot | install certbot
     package:
       name: certbot