Use explicit wildcard over empty

Added in hyprwm/hyprpaper#315 also adds
mkDefault in order to allow users to override values

Author: noisesfromspace

modules/hyprpaper/hm.nix

@@ -4,7 +4,7 @@ mkTarget {
     { image }:
     {
       services.hyprpaper.settings.wallpaper = lib.singleton {
-        monitor = "";
+        monitor = lib.mkDefault "*";
         path = toString image;
       };
     };