make sure variables are defined before parsing value

roles/database/tasks/main.yml

@@ -12,6 +12,10 @@
       - mysql is defined
       - mysql == true
   - import_tasks: install_postgres.yml
-    when: postgres == true
+    when:
+      - postgres is defined
+      - postgres == true
   - import_tasks: install_redis.yml
-    when: redis == true
+    when:
+      - redis is defined
+      - redis == true

roles/nameserver/tasks/main.yml

@@ -6,7 +6,10 @@
   - block:
     - include_tasks: install_unbound.yml
     - include_tasks: configure_unbound_snmpd.yml
-    when: unbound == true and powerdns_recursor != true
+    when:
+      - unbound is defined
+      - unbound == true
+      - powerdns is not defined or powerdns_recursor != true
 
   - name: nameserver | unbound | disable systemd-resolved
     include_tasks: disable-systemd-resolved.yml
@@ -15,12 +18,18 @@
   - block:
     - include_tasks: install_bind.yml
     - include_tasks: configure_bind_snmpd.yml
-    when: bind == true and powerdns_auth != true
+    when:
+      - bind is defined
+      - bind == true
+      - powerdns_auth is not defined or powerdns_auth != true
 
   - block:
     - include_tasks: install_powerdns_recursor.yml
     - include_tasks: configure_powerdns_recursor_snmpd.yml
-    when: powerdns_recursor == true and unbound != true
+    when:
+      - powerdns_recursor is defined
+      - powerdns_recursor == true
+      - unbound is not defined or unbound != true
 
   rescue:
     - set_fact: task_failed=true