sane-vpn: fix so that sane-vpn do works even without hickory-dns installed

pkgs/by-name/sane-scripts/src/sane-vpn

@@ -294,6 +294,11 @@ stores = {{ type = "forward", name_servers = [
 '''
     elif dir_ == ToggleDir.Down:
         text = ""
+
+    if not os.path.isdir("/var/lib/hickory-dns"):
+        logger.info("not restarting hickory-dns because it appears to not be in use")
+        return
+
     with open("/var/lib/hickory-dns/dhcp-configs/sane-vpn.toml", "w") as f:
         f.write(text)
     subprocess.check_call([ "systemctl", "restart", "hickory-dns-localhost" ])