[2.0.0] NPE when get subscribers by NamingSubscriberServiceLocalImpl

[pixystone]

Describe the bug

NamingSubscriberServiceLocalImpl：

@Override
    public Collection<Subscriber> getSubscribers(String namespaceId, String serviceName) {
        Collection<Subscriber> result = new HashSet<>();
        result.addAll(namingSubscriberServiceV1.getSubscribers(namespaceId, serviceName));
        result.addAll(namingSubscriberServiceV2.getSubscribers(namespaceId, serviceName));
        return result;
    }

NamingSubscriberServiceV1Impl：

@Override
    public Collection<Subscriber> getSubscribers(String namespaceId, String serviceName) {
        String serviceKey = UtilsAndCommons.assembleFullServiceName(namespaceId, serviceName);
        ConcurrentMap<String, PushClient> clientConcurrentMap = clientMap.get(serviceKey);
      
        if (Objects.isNull(clientConcurrentMap)) {
            return null;             // <---
        }
        ...
    }

Expected behavior

returns an empty list

Acutally behavior

return null

Desktop (please complete the following information):

• Version [nacos-server 2.0.0-BETA]
• Module [naming]