[Bug] dubbo Provider列表变更通知consumer 消费端 出现NullPointerException bug

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### Search before asking

- [X] I had searched in the [issues](https://github.com/apache/dubbo/issues?q=is%3Aissue) and found no similar issues.

### Apache Dubbo Component

Java SDK (apache/dubbo)

### Dubbo Version

Dubbo 3.2.11 JDK 17

### Steps to reproduce this issue

上线发布 Dubbo 服务提供者，服务提供者和消费者都只有一个POD实例

### What you expected to happen

_time_: 2024-05-16T10:17:12.878479339+08:00
content: 2024-05-16 10:17:12,877 ERROR [nacos.publisher-com.alibaba.nacos.client.naming.event.InstancesChangeEvent] [TRACEID:] [c.a.n.c.n.NotifyCenter:208]}: Event callback exception:
java.lang.NullPointerException: null
at java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1693) ~[?:?]
at org.apache.dubbo.registry.client.DefaultServiceInstance.toURL(DefaultServiceInstance.java:294) ~[dubbo-3.2.11.jar!/:3.2.11]
at org.apache.dubbo.registry.client.event.listener.ServiceInstancesChangedListener.getServiceUrlsCache(ServiceInstancesChangedListener.java:407) ~[dubbo-3.2.11.jar!/:3.2.11]
at org.apache.dubbo.registry.client.event.listener.ServiceInstancesChangedListener.lambda$doOnEvent$5(ServiceInstancesChangedListener.java:234) ~[dubbo-3.2.11.jar!/:3.2.11]
at java.util.HashMap.computeIfAbsent(HashMap.java:1220) ~[?:?]
at org.apache.dubbo.registry.client.event.listener.ServiceInstancesChangedListener.doOnEvent(ServiceInstancesChangedListener.java:232) ~[dubbo-3.2.11.jar!/:3.2.11]
at org.apache.dubbo.registry.client.event.listener.ServiceInstancesChangedListener.onEvent(ServiceInstancesChangedListener.java:125) ~[dubbo-3.2.11.jar!/:3.2.11]
at org.apache.dubbo.registry.nacos.NacosServiceDiscovery.handleEvent(NacosServiceDiscovery.java:245) ~[dubbo-3.2.11.jar!/:3.2.11]
at org.apache.dubbo.registry.nacos.NacosServiceDiscovery.access$000(NacosServiceDiscovery.java:65) ~[dubbo-3.2.11.jar!/:3.2.11]
at org.apache.dubbo.registry.nacos.NacosServiceDiscovery$NacosEventListener.onEvent(NacosServiceDiscovery.java:217) ~[dubbo-3.2.11.jar!/:3.2.11]
at com.alibaba.nacos.client.naming.event.InstancesChangeNotifier.onEvent(InstancesChangeNotifier.java:126) ~[nacos-client-2.2.1.jar!/:?]
at com.alibaba.nacos.client.naming.event.InstancesChangeNotifier.onEvent(InstancesChangeNotifier.java:42) ~[nacos-client-2.2.1.jar!/:?]
at com.alibaba.nacos.common.notify.DefaultPublisher.lambda$notifySubscriber$0(DefaultPublisher.java:199) ~[nacos-client-2.2.1.jar!/:?]
at com.alibaba.nacos.common.notify.DefaultPublisher.notifySubscriber(DefaultPublisher.java:206) [nacos-client-2.2.1.jar!/:?]
at com.alibaba.nacos.common.notify.DefaultPublisher.receiveEvent(DefaultPublisher.java:190) [nacos-client-2.2.1.jar!/:?]
at com.alibaba.nacos.common.notify.DefaultPublisher.openEventHandler(DefaultPublisher.java:112) [nacos-client-2.2.1.jar!/:?]
at com.alibaba.nacos.common.notify.DefaultPublisher.run(DefaultPublisher.java:95) [nacos-client-2.2.1.jar!/:?]

### Anything else

_No response_

### Are you willing to submit a pull request to fix on your own?

- [X] Yes I am willing to submit a pull request on my own!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Reproduce the provider-list change with one provider and one consumer POD on Dubbo 3.2.11 with JDK 17. Start at DefaultServiceInstance.toURL and follow ServiceInstancesChangedListener.getServiceUrlsCache, doOnEvent, and NacosServiceDiscovery.handleEvent from the reported stack trace. Done means the consumer processes the provider change without the reported NullPointerException.

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