systemctl: don't update the reboot parameter if none is given

Otherwise systemd-reboot.service will remove the parameter that was set
before. This was broken in commit "b986229efe2cc96157aa14c37bab7843311bbef1
systemctl: bugfix for systemctl reboot command with argument"

(cherry picked from commit c31c4324c5)

src/systemctl/systemctl.c

@@ -3031,7 +3031,7 @@ static int start_special(sd_bus *bus, char **args) {
                 return -EPERM;
         }
 
-        if (a == ACTION_REBOOT) {
+        if (a == ACTION_REBOOT && args[1]) {
                 r = update_reboot_param_file(args[1]);
                 if (r < 0)
                         return r;