ipn/ipnlocal: send vipServices info via c2n even it's incomplete (#15166)

This commit updates the logic of vipServicesFromPrefsLocked, so that it would return the vipServices list
even when service host is only advertising the service but not yet serving anything. This makes control
always get accurate state of service host in terms of serving a service.

Fixes tailscale/corp#26843

Signed-off-by: KevinLiang10 <37811973+KevinLiang10@users.noreply.github.com>

ipn/ipnlocal/local.go

@@ -8222,15 +8222,13 @@ func (b *LocalBackend) vipServiceHash(services []*tailcfg.VIPService) string {
 func (b *LocalBackend) vipServicesFromPrefsLocked(prefs ipn.PrefsView) []*tailcfg.VIPService {
 	// keyed by service name
 	var services map[tailcfg.ServiceName]*tailcfg.VIPService
-	if !b.serveConfig.Valid() {
-		return nil
-	}
-
-	for svc, config := range b.serveConfig.Services().All() {
-		mak.Set(&services, svc, &tailcfg.VIPService{
-			Name:  svc,
-			Ports: config.ServicePortRange(),
-		})
+	if b.serveConfig.Valid() {
+		for svc, config := range b.serveConfig.Services().All() {
+			mak.Set(&services, svc, &tailcfg.VIPService{
+				Name:  svc,
+				Ports: config.ServicePortRange(),
+			})
+		}
 	}
 
 	for _, s := range prefs.AdvertiseServices().All() {