gnome: only exclude GNOME backgrounds when setting wallpaper (#2088)

Link: https://github.com/nix-community/stylix/pull/2088

Reviewed-by: NAHO <90870942+trueNAHO@users.noreply.github.com>
(cherry picked from commit f6e9fbaeb3)

modules/gnome/nixos.nix

@@ -29,12 +29,14 @@ in
         )
       )
       {
-        # As Stylix is controlling the wallpaper, there is no need for this
+        # If Stylix is controlling the wallpaper, there is no need for this
         # pack of default wallpapers to be installed.
         # If you want to use one, you can set stylix.image to something like
         # "${pkgs.gnome-backgrounds}/path/to/your/preferred/background"
         # which will then download the pack regardless of its exclusion below.
-        environment.gnome.excludePackages = [ pkgs.gnome-backgrounds ];
+        environment.gnome.excludePackages = lib.mkIf (config.stylix.image != null) [
+          pkgs.gnome-backgrounds
+        ];
 
         nixpkgs.overlays = [
           (_: super: {