systemctl: fail in the case that no unit files were found

Previously systemctl died with message

-bash-4.2# systemctl --root /rawhi list-unit-files
(src/systemctl/systemctl.c:868) Out of memory.

in the case that no unit files were found in the --root
or the directory did not exist.

So lets return ENOENT in the case that --root does not exist
and empty list in the case that there are no unit files.

src/systemctl/systemctl.c

@@ -1350,6 +1350,10 @@ static int list_unit_files(sd_bus *bus, char **args) {
                 }
 
                 n_units = hashmap_size(h);
+
+                if (n_units == 0)
+                        return 0;
+
                 units = new(UnitFileList, n_units);
                 if (!units) {
                         unit_file_list_free(h);