Only run if {{ db }} is defined

plays/lib/database.yml

@@ -3,6 +3,7 @@
   set_fact:
     mount_map:
       mariadb: /var/lib/mysql
+  when: db is defined
 
 - name: "Create {{ item.type }} database"
   no_log: true
@@ -16,6 +17,7 @@
   args:
     creates: "/var/lib/dokku/services/{{ item.type }}/{{ app }}"
   with_items: "{{ db }}"
+  when: db is defined
 
 - name: "Link {{ item.type }} database to application"
   dokku_service_link:
@@ -23,6 +25,7 @@
     name: "{{ app }}"
     service: "{{ item.type }}"
   with_items: "{{ db }}"
+  when: db is defined
 
 - name: "Specify {{ item.type }} volume mounts"
   dokku_storage:
@@ -30,3 +33,4 @@
     mounts:
       - "/var/lib/dokku/services/{{ item.type }}/{{ app }}:{{ mount_map[item.type] }}"
   with_items: "{{ db }}"
+  when: db is defined

plays/post_delete.yml

@@ -13,4 +13,4 @@
       args:
         removes: "/var/lib/dokku/services/{{ item.type }}/{{ app }}"
       with_items: "{{ db }}"
-      when: db exists
+      when: db is defined