Refactor ES addition logic in Destination

This is a second take on #12427, which avoided a theoretical/correctness
issue around overwritting new ES addresses with stale data.

We had to revert that in #12589 because the change introduced a bug, by
returning early when the ES had no addresses and failed to properly
initiallize `addesses` for the portPublisher.

This just removes the early return.

controller/api/destination/watcher/endpoints_watcher.go

@@ -795,17 +795,22 @@ func (pp *portPublisher) updateEndpoints(endpoints *corev1.Endpoints) {
 func (pp *portPublisher) addEndpointSlice(slice *discovery.EndpointSlice) {
 	newAddressSet := pp.endpointSliceToAddresses(slice)
 	for id, addr := range pp.addresses.Addresses {
-		newAddressSet.Addresses[id] = addr
-		newAddressSet.LocalTrafficPolicy = pp.localTrafficPolicy
+		if _, ok := newAddressSet.Addresses[id]; !ok {
+			newAddressSet.Addresses[id] = addr
+		}
 	}
 
 	add, _ := diffAddresses(pp.addresses, newAddressSet)
-	if len(add.Addresses) > 0 {
-		for _, listener := range pp.listeners {
-			listener.Add(add)
-		}
+
+	for _, listener := range pp.listeners {
+		listener.Add(add)
 	}
 
+	// even if the ES doesn't have addresses yet we need to create a new
+	// pp.addresses entry with the appropriate Labels and LocalTrafficPolicy,
+	// which isn't going to be captured during the ES update event when
+	// addresses get added
+
 	pp.addresses = newAddressSet
 	pp.exists = true
 	pp.metrics.incUpdates()