[I] [BUG] 通过nacos同步应用时，当只有一个服务实例，重启时会偶现"com.alibaba.nacos.api.naming.po jo.Instance.getInstanceId()" is null [shenyu]

[via GitHub]

BraveheartStone opened a new issue, #6143:
URL: https://github.com/apache/shenyu/issues/6143

   ### Is there an existing issue for this?
   
   - [x] I have searched the existing issues
   
   ### Current Behavior
   
   通过nacos发现服务时，服务只有一个实例的情况下，重启下游服务，会偶现nacos实例为空的情况。导致通过shenyu调用下游服务时报错：
   ```
   "Can not find healthy upstream url, please check your configuration!"
   ```
   
   ### Expected Behavior
   
   只有一个服务重新完成后，能够正常识别应用。
   
   ### Steps To Reproduce
   
   1. Divide中添加 Selector，通过 nacos 发现服务
   2. 下游服务只有一个实例
   3. 重启下游服务时，偶现上面错误
   
   ### Environment
   
   ```markdown
   ShenYu version(s):2.7.0.2
   ```
   
   ### Debug logs
   
   错误日志：
   ``` java
   [nacos.publisher-com.alibaba.nacos.client.naming.event.InstancesChangeEvent] 
ERROR
    com.alibaba.nacos.common.notify.NotifyCenter - Event callback exception: 
   java.lang.NullPointerException: Cannot invoke "String.equals(Object)" 
because the return value of "com.alibaba.nacos.api.naming.po
   jo.Instance.getInstanceId()" is null
           at 
org.apache.shenyu.registry.nacos.NacosInstanceRegisterRepository.lambda$compareInstances$3(NacosInstanceRegisterRepository.ja
   va:188)
           at 
java.base/java.util.stream.MatchOps$1MatchSink.accept(MatchOps.java:90)
           at 
java.base/java.util.ArrayList$ArrayListSpliterator.tryAdvance(ArrayList.java:1602)
           at 
java.base/java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:129)
           at 
java.base/java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:527)
           at 
java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:513)
           at 
java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
           at 
java.base/java.util.stream.MatchOps$MatchOp.evaluateSequential(MatchOps.java:230)
           at 
java.base/java.util.stream.MatchOps$MatchOp.evaluateSequential(MatchOps.java:196)
           at 
java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
           at 
java.base/java.util.stream.ReferencePipeline.anyMatch(ReferencePipeline.java:632)
           at 
org.apache.shenyu.registry.nacos.NacosInstanceRegisterRepository.lambda$compareInstances$4(NacosInstanceRegisterRepository.ja
   va:188)
           at 
java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:178)
           at 
java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1625)
           at 
java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
           at 
java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
           at 
java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:921)
           at 
java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
           at 
java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:682)
           at 
org.apache.shenyu.registry.nacos.NacosInstanceRegisterRepository.compareInstances(NacosInstanceRegisterRepository.java:189)
           at 
org.apache.shenyu.registry.nacos.NacosInstanceRegisterRepository.lambda$watchInstances$0(NacosInstanceRegisterRepository.java
   :135)
           at 
com.alibaba.nacos.client.naming.event.InstancesChangeNotifier.onEvent(InstancesChangeNotifier.java:126)
           at 
com.alibaba.nacos.client.naming.event.InstancesChangeNotifier.onEvent(InstancesChangeNotifier.java:42)
           at 
com.alibaba.nacos.common.notify.DefaultPublisher.lambda$notifySubscriber$0(DefaultPublisher.java:199)
           at 
com.alibaba.nacos.common.notify.DefaultPublisher.notifySubscriber(DefaultPublisher.java:206)
           at 
com.alibaba.nacos.common.notify.DefaultPublisher.receiveEvent(DefaultPublisher.java:190)
           at 
com.alibaba.nacos.common.notify.DefaultPublisher.openEventHandler(DefaultPublisher.java:112)
           at 
com.alibaba.nacos.common.notify.DefaultPublisher.run(DefaultPublisher.java:95)
   ```
   
   ### Anything else?
   
   建议修改后的方案：
   org/apache/shenyu/registry/nacos/NacosInstanceRegisterRepository.java:188 :
   
   ```java
           Set<Instance> updatedInstances = currentInstances.stream()
               .filter(currentInstance -> currentInstance.getInstanceId() != 
null &&
                   previousInstances.stream()
                       .anyMatch(previousInstance ->
                           previousInstance.getInstanceId() != null &&
                               
currentInstance.getInstanceId().equals(previousInstance.getInstanceId()) &&
                               !currentInstance.equals(previousInstance)))
               .collect(Collectors.toSet());
   ```


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