Fix log spam when there is any DNS based LoadBalancer Service

IP in Service.Status.LoadBalancer.Ingress is optional and may be not set
for load-balancer ingress points that are DNS based. It should check if
the IP is empty before appending it to the slice, otherwise every round
of sync will produce one error log "Skipping invalid IP:" for each IP,
leading to log spam.

The same issue has been fixed in K8s upstream by
kubernetes/kubernetes#101690.

Signed-off-by: Quan Tian <qtian@vmware.com>

third_party/proxy/service.go

@@ -207,7 +207,10 @@ func (sct *ServiceChangeTracker) newBaseServiceInfo(port *v1.ServicePort, servic
 	// Obtain Load Balancer Ingress IPs
 	var ips []string
 	for _, ing := range service.Status.LoadBalancer.Ingress {
-		ips = append(ips, ing.IP)
+		// IP is optional and may be not set for load-balancer ingress points that are DNS based.
+		if ing.IP != "" {
+			ips = append(ips, ing.IP)
+		}
 	}
 
 	if len(ips) > 0 {
@@ -286,8 +289,10 @@ func NewServiceChangeTracker(makeServiceInfo makeServicePortFunc,
 // otherwise return false.  Update can be used to add/update/delete items of ServiceChangeMap.  For example,
 // Add item
 //   - pass <nil, service> as the <previous, current> pair.
+//
 // Update item
 //   - pass <oldService, service> as the <previous, current> pair.
+//
 // Delete item
 //   - pass <service, nil> as the <previous, current> pair.
 func (sct *ServiceChangeTracker) Update(previous, current *v1.Service) bool {
@@ -409,17 +414,18 @@ func (sm *ServiceMap) apply(changes *ServiceChangeTracker, UDPStaleClusterIP set
 // tell if a service is deleted or updated.
 // The returned value is one of the arguments of ServiceMap.unmerge().
 // ServiceMap A Merge ServiceMap B will do following 2 things:
-//   * update ServiceMap A.
-//   * produce a string set which stores all other ServiceMap's ServicePortName.String().
+//   - update ServiceMap A.
+//   - produce a string set which stores all other ServiceMap's ServicePortName.String().
+//
 // For example,
 //   - A{}
 //   - B{{"ns", "cluster-ip", "http"}: {"172.16.55.10", 1234, "TCP"}}
-//     - A updated to be {{"ns", "cluster-ip", "http"}: {"172.16.55.10", 1234, "TCP"}}
-//     - produce string set {"ns/cluster-ip:http"}
+//   - A updated to be {{"ns", "cluster-ip", "http"}: {"172.16.55.10", 1234, "TCP"}}
+//   - produce string set {"ns/cluster-ip:http"}
 //   - A{{"ns", "cluster-ip", "http"}: {"172.16.55.10", 345, "UDP"}}
 //   - B{{"ns", "cluster-ip", "http"}: {"172.16.55.10", 1234, "TCP"}}
-//     - A updated to be {{"ns", "cluster-ip", "http"}: {"172.16.55.10", 1234, "TCP"}}
-//     - produce string set {"ns/cluster-ip:http"}
+//   - A updated to be {{"ns", "cluster-ip", "http"}: {"172.16.55.10", 1234, "TCP"}}
+//   - produce string set {"ns/cluster-ip:http"}
 func (sm *ServiceMap) merge(other ServiceMap) sets.String {
 	// existingPorts is going to store all identifiers of all services in `other` ServiceMap.
 	existingPorts := sets.NewString()