fix(core): keep the system resolver fallback when no DNS servers are reported

An Android service with no DNS servers sends an empty list, which split into
a single empty address that mihomo builds into a "udp://:" nameserver. The
system resolver then counted it as configured and never fell back to its
default nameservers, so lookups through it failed while the preferred network
reported no DNS.

Changelog: DNS lookups no longer fail on networks that report no DNS servers

core/lib.go

@@ -246,6 +246,12 @@ var (
 )
 
 func handleUpdateDns(value string) {
+	// mihomo turns the [""] that Split returns for no servers into a "udp://:"
+	// nameserver, which keeps the system resolver off its default fallback.
+	var addr []string
+	if value != "" {
+		addr = strings.Split(value, ",")
+	}
 	seq := dnsUpdateSeq.Add(1)
 	safeGoDetached("updateDns", func() {
 		dnsUpdateMu.Lock()
@@ -254,7 +260,7 @@ func handleUpdateDns(value string) {
 			return
 		}
 		log.Infoln("[DNS] updateDns %s", value)
-		dns.UpdateSystemDNS(strings.Split(value, ","))
+		dns.UpdateSystemDNS(addr)
 		dns.FlushCacheWithDefaultResolver()
 	})
 }