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: 9ba20645b0 ("luci-proto-wireguard: rewrite protocol handler")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

protocols/luci-proto-wireguard/htdocs/luci-static/resources/protocol/wireguard.js

@@ -686,8 +686,8 @@ return network.registerProtocol('wireguard', {
 			return Promise.all([
 				network.getWANNetworks(),
 				network.getWAN6Networks(),
-				uci.load('ddns'),
-				uci.load('system'),
+				L.resolveDefault(uci.load('ddns')),
+				L.resolveDefault(uci.load('system')),
 				parent.save(null, true)
 			]).then(function(data) {
 				var hostnames = [];