hyprpaper: fix service when no config file

The systemd user service depends on

  config.xdg.configFile."hypr/hyprpaper.conf".source

for `X-Restart-Triggers`. When `cfg.settings` is the default `{}`,
this causes failure since

  config.xdg.configFile."hypr/hyprpaper.conf".source

will not exist.

Making the addition conditional on `cfg.settings` actually having
content, which would mean `xdg.configFile."hypr/hyprpaper.conf"` does
exist, avoids the error.

modules/services/hyprpaper.nix

@@ -75,7 +75,7 @@ in {
         Description = "hyprpaper";
         After = [ "graphical-session-pre.target" ];
         PartOf = [ "graphical-session.target" ];
-        X-Restart-Triggers =
+        X-Restart-Triggers = mkIf (cfg.settings != { })
           [ "${config.xdg.configFile."hypr/hyprpaper.conf".source}" ];
       };
 

tests/modules/services/hyprpaper/default.nix

@@ -1 +1,4 @@
-{ hyprpaper-basic-configuration = ./basic-configuration.nix; }
+{
+  hyprpaper-basic-configuration = ./basic-configuration.nix;
+  hyprpaper-no-configuration = ./no-configuration.nix;
+}

tests/modules/services/hyprpaper/no-configuration.nix

@@ -0,0 +1,17 @@
+{ pkgs, ... }:
+
+{
+  services.hyprpaper = {
+    enable = true;
+    settings = { };
+  };
+
+  test.stubs.hyprpaper = { };
+
+  nmt.script = ''
+    config=home-files/.config/hypr/hyprpaper.conf
+    clientServiceFile=home-files/.config/systemd/user/hyprpaper.service
+    assertPathNotExists $config
+    assertFileExists $clientServiceFile
+  '';
+}