fw4: prevent null access when no ipsets are defined

When rules are configured which reference ipsets and no ipsets are declared
at all, fw4 crashed with a null reference exception while validating the
rule.

Solve the issue by using the optional chaining operator on the potentially
null filter result.

Ref: https://forum.openwrt.org/t/x/144176
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

root/usr/share/ucode/fw4.uc

@@ -2305,7 +2305,7 @@ return {
 		let ipset;
 
 		if (rule.ipset) {
-			ipset = filter(this.state.ipsets, s => (s.name == rule.ipset.name))[0];
+			ipset = filter(this.state.ipsets, s => (s.name == rule.ipset.name))?.[0];
 
 			if (!ipset) {
 				this.warn_section(data, `references unknown set '${rule.ipset.name}'`);
@@ -2605,7 +2605,7 @@ return {
 		let ipset;
 
 		if (redir.ipset) {
-			ipset = filter(this.state.ipsets, s => (s.name == redir.ipset.name))[0];
+			ipset = filter(this.state.ipsets, s => (s.name == redir.ipset.name))?.[0];
 
 			if (!ipset) {
 				this.warn_section(data, `references unknown set '${redir.ipset.name}'`);