fix NPE

fix NPE

spring-cloud-alibaba-nacos-discovery/src/main/java/org/springframework/cloud/alibaba/nacos/ribbon/NacosRule.java

@@ -36,10 +36,13 @@ public class NacosRule extends AbstractLoadBalancerRule {
             String name = loadBalancer.getName();
 
             NamingService namingService = this.nacosDiscoveryProperties.namingServiceInstance();
-
             List<Instance> instances = namingService.selectInstances(name, true);
-            List<Instance> instancesToChoose = instances;
+            if (CollectionUtils.isEmpty(instances)) {
+                log.warn("{}服务当前无任何实例", name);
+                return null;
+            }
 
+            List<Instance> instancesToChoose = instances;
             if (StringUtils.isNotBlank(clusterName)) {
                 List<Instance> sameClusterInstances = instances.stream()
                         .filter(instance -> Objects.equals(clusterName, instance.getClusterName()))