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/ptr_lookup.go

@@ -41,6 +41,9 @@ func (p *ptrDiscover) String() string {
 }
 
 func (p *ptrDiscover) List() []string {
+	if p == nil {
+		return nil
+	}
 	var ips []string
 	p.hostname.Range(func(key, value any) bool {
 		ips = append(ips, key.(string))