nixos/paperless: add missing default to domain (#439217)

nixos/modules/services/misc/paperless.nix

···
 }:
 let
   cfg = config.services.paperless;
+  opt = options.services.paperless;
 
   defaultUser = "paperless";
   defaultFont = "${pkgs.liberation_ttf}/share/fonts/truetype/LiberationSerif-Regular.ttf";
···
     };
 
     domain = lib.mkOption {
-      type = lib.types.str;
+      type = with lib.types; nullOr str;
+      default = null;
       example = "paperless.example.com";
       description = "Domain under which paperless will be available.";
     };
···
   config = lib.mkIf cfg.enable (
     lib.mkMerge [
       {
+        assertions = [
+          {
+            assertion = cfg.configureNginx -> cfg.domain != null;
+            message = "${opt.configureNginx} requires ${opt.domain} to be configured.";
+          }
+        ];
+
         services.paperless.manage = manage;
         environment.systemPackages = [ manage ];
 
···
         };
 
         services.paperless.settings = lib.mkMerge [
-          (lib.mkIf (cfg.domain != "") {
+          (lib.mkIf (cfg.domain != null) {
             PAPERLESS_URL = "https://${cfg.domain}";
           })
           (lib.mkIf cfg.database.createLocally {