linux-wallpaperengine: fix evaluation error when passing null (#7161)

modules/services/linux-wallpaperengine.nix

@@ -19,7 +19,8 @@ in
     package = lib.mkPackageOption pkgs "linux-wallpaperengine" { };
 
     assetsPath = mkOption {
-      type = types.path;
+      type = types.nullOr types.path;
+      default = null;
       description = "Path to the assets directory.";
     };
 
@@ -104,6 +105,10 @@ in
   config = lib.mkIf cfg.enable {
     assertions = [
       (lib.hm.assertions.assertPlatform "services.linux-wallpaperengine" pkgs lib.platforms.linux)
+      ({
+        assertion = cfg.wallpapers != null;
+        message = "linux-wallpaperengine: You must set at least one wallpaper";
+      })
     ];
 
     home.packages = [ cfg.package ];
@@ -135,8 +140,8 @@ in
         Service = {
           ExecStart =
             lib.getExe cfg.package
-            + " --assets-dir ${cfg.assetsPath} "
-            + "--clamping ${cfg.clamping} "
+            + (lib.optionalString (cfg.assetsPath != null) " --assets-dir ${cfg.assetsPath} ")
+            + (lib.optionalString (cfg.clamping != null) "--clamping ${cfg.clamping} ")
             + (lib.strings.concatStringsSep " " args);
           Restart = "on-failure";
         };

tests/modules/services/linux-wallpaperengine/default.nix

@@ -1 +1,4 @@
-{ linux-wallpaperengine-basic-configuration = ./basic-configuration.nix; }
+{
+  linux-wallpaperengine-basic-configuration = ./basic-configuration.nix;
+  linux-wallpaperengine-null-options = ./null-options.nix;
+}

tests/modules/services/linux-wallpaperengine/null-options-expected.service

@@ -0,0 +1,11 @@
+[Install]
+WantedBy=graphical-session.target
+
+[Service]
+ExecStart=@linux-wallpaperengine@/bin/linux-wallpaperengine
+Restart=on-failure
+
+[Unit]
+After=graphical-session.target
+Description=Implementation of Wallpaper Engine on Linux
+PartOf=graphical-session.target

tests/modules/services/linux-wallpaperengine/null-options.nix

@@ -0,0 +1,11 @@
+{
+  services.linux-wallpaperengine = {
+    enable = true;
+  };
+
+  nmt.script = ''
+    assertFileContent \
+        home-files/.config/systemd/user/linux-wallpaperengine.service \
+        ${./null-options-expected.service}
+  '';
+}