luci-proto-wireguard: gracefully deal with missing uci configs

The /etc/config/ddns in particular might not be present on the system,
don't fail if it is absent.

Fixes: #5838
Fixes: 9ba20645

 ("luci-proto-wireguard: rewrite protocol handler")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>