Fixes panic when nonexistent node is queried

.changelog/595.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+Fix a nil pointer panic when Consul returns a literal "null" when checking catalog nodes.
+```

internal/k8s/service/resolver.go

@@ -337,6 +337,9 @@ func (r *backendResolver) findGlobalCatalogService(service *corev1.Service) (*Re
 				r.logger.Trace("error retrieving node services", "error", err, "node", node.Node)
 				return nil, err
 			}
+			if nodeWithServices == nil {
+				continue
+			}
 			if len(nodeWithServices.Services) == 0 {
 				continue
 			}