xds/e2e: Use Metadata field from BackendOptions only when it is non-nil (#8619)

Fixes: #8545 

Check BackendOptions.Metadata field before assigning it to
`v3corepb.Metadata`.

RELEASE NOTES: N/A

Author: hugehoo

internal/testutils/xds/e2e/clientresources.go

@@ -712,11 +712,7 @@ func EndpointResourceWithOptions(opts EndpointOptions) *v3endpointpb.ClusterLoad
 					},
 				}
 			}
-			metadata, err := structpb.NewStruct(b.Metadata)
-			if err != nil {
-				panic(err)
-			}
-			lbEndpoints = append(lbEndpoints, &v3endpointpb.LbEndpoint{
+			lbEndpoint := &v3endpointpb.LbEndpoint{
 				HostIdentifier: &v3endpointpb.LbEndpoint_Endpoint{Endpoint: &v3endpointpb.Endpoint{
 					Address: &v3corepb.Address{Address: &v3corepb.Address_SocketAddress{
 						SocketAddress: &v3corepb.SocketAddress{
@@ -729,12 +725,20 @@ func EndpointResourceWithOptions(opts EndpointOptions) *v3endpointpb.ClusterLoad
 				}},
 				HealthStatus:        b.HealthStatus,
 				LoadBalancingWeight: &wrapperspb.UInt32Value{Value: b.Weight},
-				Metadata: &v3corepb.Metadata{
+			}
+
+			if b.Metadata != nil {
+				metadata, err := structpb.NewStruct(b.Metadata)
+				if err != nil {
+					panic(fmt.Sprintf("failed to marshal metadata: %v", err))
+				}
+				lbEndpoint.Metadata = &v3corepb.Metadata{
 					FilterMetadata: map[string]*structpb.Struct{
 						"envoy.lb": metadata,
 					},
-				},
-			})
+				}
+			}
+			lbEndpoints = append(lbEndpoints, lbEndpoint)
 		}
 
 		l := locality.Locality