ipn: bail out a bit earlier when a peer doesn't have a DNS name

It's properly handled later in tsdns.NewMap anyway, but there's work
done in the meantime that can be skipped when a peer lacks a DNS name.
It's also more clear that it's okay for it to be blank.

ipn/local.go

@@ -555,7 +555,7 @@ func (b *LocalBackend) updateDNSMap(netMap *controlclient.NetworkMap) {
 
 	nameToIP := make(map[string]netaddr.IP)
 	set := func(name string, addrs []wgcfg.CIDR) {
-		if len(addrs) == 0 {
+		if len(addrs) == 0 || name == "" {
 			return
 		}
 		nameToIP[name] = netaddr.IPFrom16(addrs[0].IP.Addr)