nixos/gitlab: Fix error when GitLab Pages is not enabled

Even if GitLab is used without Pages, some of its settings are still
used in the gitlab.yml file, resulting in errors such as

error: The option `services.gitlab.pages.settings.pages-domain' is used but not defined.

To fix this, make the settings' usage conditional on Pages being
enabled.

nixos/modules/services/misc/gitlab.nix

@@ -156,7 +156,7 @@ let
       };
       extra = {};
       uploads.storage_path = cfg.statePath;
-      pages = {
+      pages = optionalAttrs cfg.pages.enable {
         enabled = cfg.pages.enable;
         port = 8090;
         host = cfg.pages.settings.pages-domain;