systemd: fix sd-switch error on empty target directory

After update to sd-switch 0.5.0, `home-manager switch` failed with the
following error message:

> Error: Error switching
>
> Caused by:
>     0: Failed to build switch plan
>     1: No such file or directory (os error 2)

This was caused by a non-existing target unit directory due to no user
services being managed by home-manager. This change makes sd-switch
run with pkgs.emptyDirectory as the target in such cases.

Closes: https://github.com/nix-community/home-manager/issues/5552

modules/systemd.nix

@@ -336,7 +336,7 @@ in {
           ${pkgs.sd-switch}/bin/sd-switch \
             ''${DRY_RUN:+--dry-run} $VERBOSE_ARG ${timeoutArg} \
             ''${oldGenPath:+--old-units $oldGenPath/home-files/.config/systemd/user} \
-            --new-units $newGenPath/home-files/.config/systemd/user
+            --new-units "$newUnitsDir"
         '';
       };
 
@@ -354,8 +354,15 @@ in {
           warnEcho "Attempting to reload services anyway..."
         fi
 
+        newUnitsDir="$newGenPath/home-files/.config/systemd/user"
+        if [[ ! -e $newUnitsDir ]]; then
+          newUnitsDir=${pkgs.emptyDirectory}
+        fi
+
         ${ensureRuntimeDir} \
           ${getAttr cfg.startServices cmd}
+
+        unset newUnitsDir
       else
         echo "User systemd daemon not running. Skipping reload."
       fi