database: Don't add targets if no dbs are assigned

Having unnecessary target units for every container is distracting for
users, so let's actually make sure that these targets only exist
whenever there are databases assigned.

Signed-off-by: aszlig <aszlig@nix.build>
Issue: #12

modules/web/database/default.nix

@@ -122,7 +122,7 @@ in {
     '';
   };
 
-  config = lib.mkIf config.enable {
+  config = lib.mkIf (config.enable && config.database != {}) {
     systemd.targets = {
       db-server = {
         description = "Database Management Systems For ${config.uniqueName}";