luci-base: firewall.js: gracefully handle missing uci configuration

Ref: https://forum.openwrt.org/t/luci-rpc-error/61760
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit a8d85ed71a)

modules/luci-base/htdocs/luci-static/resources/firewall.js

@@ -5,7 +5,7 @@
 
 
 function initFirewallState() {
-	return uci.load('firewall');
+	return L.resolveDefault(uci.load('firewall'));
 }
 
 function parseEnum(s, values) {