all: include client IP if ctrld is dnsmasq upstream

So ctrld can record the raw/original client IP instead of looking up
from MAC to IP, which may not the right choice in some network setup
like using wireguard/vpn on Merlin router.

internal/clientinfo/mdns.go

@@ -48,6 +48,9 @@ func (m *mdns) String() string {
 }
 
 func (m *mdns) List() []string {
+	if m == nil {
+		return nil
+	}
 	var ips []string
 	m.name.Range(func(key, value any) bool {
 		ips = append(ips, key.(string))