Do not deploy the empty kanshi config

defaults/xdg/kanshi/config is a 0-byte placeholder, unused on Arch as
well, where kanshi reads ~/.config/kanshi/config instead. Deploying it
made kanshi fail to parse and restart in a loop. The unit now starts only
when a user config exists.

modules/services.nix

@@ -34,6 +34,9 @@
     partOf = [ "graphical-session.target" ];
     after = [ "graphical-session.target" ];
     wantedBy = [ "graphical-session.target" ];
+    # Without a config kanshi exits with a parse error and, with
+    # Restart=on-failure, spins. Monitor profiles are per machine.
+    unitConfig.ConditionPathExists = "%h/.config/kanshi/config";
     serviceConfig = {
       Type = "simple";
       ExecStart = lib.getExe pkgs.kanshi;