systemd: avoid creating an empty user.conf

Due to the defaults in `systemd.user.settings`, the default value when
there are no settings explicitly set is `{ Manager = { }; }`. This
means an empty file is created even when `systemd.user.settings` is
never used in home-manager configuration. Since user’s `user.conf` is
preferred to the global `/etc/systemd/user.conf`, this can cause any
values set in the latter to be discarded.

modules/systemd.nix

@@ -4,7 +4,9 @@ let
 
   cfg = config.systemd.user;
 
-  inherit (lib) getAttr hm isBool literalExpression mkIf mkMerge mkOption types;
+  inherit (lib)
+    any attrValues getAttr hm isBool literalExpression mkIf mkMerge mkOption
+    types;
 
   settingsFormat = pkgs.formats.ini { listsAsDuplicateKeys = true; };
 
@@ -93,7 +95,7 @@ let
       + "\n";
   };
 
-  settings = mkIf (cfg.settings != { }) {
+  settings = mkIf (any (v: v != { }) (attrValues cfg.settings)) {
     "systemd/user.conf".source =
       settingsFormat.generate "user.conf" cfg.settings;
   };