Wallpaper: Fix non-video build.

config/Wallpaper.nix

@@ -1,13 +1,20 @@
-{ config, pkgs, ... }:
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
 let
   cfg = config.module.wallpaper;
 in
 {
-  config = {
+  config = lib.mkMerge [
+    (lib.mkIf (cfg.videoPath != null) {
       environment.systemPackages = [
         (pkgs.writeShellScriptBin "wallpaper-video" ''
           mpvpaper -o 'no-audio --loop-file --panscan=1' '*' ${cfg.videoPath}
         '')
       ];
-  };
+    })
+  ];
 }