- [ ] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.8-SNAPSHOT
* Operating System version: xxx
* Java version: xxx

### Steps to reproduce this issue

1. Do not start zookeeper server, which is used as a metadata center
2. start a provider

Pls. provide [GitHub address] to reproduce this issue.

### Expected Result

The app exit normally

### Actual Result

The app exit with a NPE

If there is an exception, please attach the exception trace:

```
Exception in thread "DubboShutdownHook" java.lang.RuntimeException: java.lang.NullPointerException
	at org.apache.dubbo.common.function.ThrowableAction.execute(ThrowableAction.java:48)
	at org.apache.dubbo.common.lang.ShutdownHookCallbacks.lambda$callback$0(ShutdownHookCallbacks.java:70)
	at java.base/java.lang.Iterable.forEach(Iterable.java:75)
	at org.apache.dubbo.common.lang.ShutdownHookCallbacks.callback(ShutdownHookCallbacks.java:70)
	at org.apache.dubbo.config.DubboShutdownHook.callback(DubboShutdownHook.java:85)
	at org.apache.dubbo.config.DubboShutdownHook.run(DubboShutdownHook.java:73)
Caused by: java.lang.NullPointerException
	at org.apache.dubbo.config.bootstrap.DubboBootstrap.unexportMetadataService(DubboBootstrap.java:1009)
	at org.apache.dubbo.config.bootstrap.DubboBootstrap.destroy(DubboBootstrap.java:1188)
	at org.apache.dubbo.config.bootstrap.DubboBootstrap$1.callback(DubboBootstrap.java:198)
	at org.apache.dubbo.common.function.ThrowableAction.execute(ThrowableAction.java:46)
	... 5 more
```

if (registryConfig.getTimeout() != null) {
            cc.setTimeout(registryConfig.getTimeout().longValue());
        }
        cc.setHighestPriority(false);
        return cc;
    }
    private void useRegistryAsMetadataCenterIfNecessary() {
        Collection<MetadataReportConfig> metadataConfigs = configManager.getMetadataConfigs();
        if (CollectionUtils.isNotEmpty(metadataConfigs)) {
            return;
        }
        configManager
                .getDefaultRegistries()
                .stream()
                .filter(this::isUsedRegistryAsMetadataCenter)
                .map(this::registryAsMetadataCenter)
                .forEach(configManager::addMetadataReport);
    }
    private boolean isUsedRegistryAsMetadataCenter(RegistryConfig registryConfig) {
        
        return Boolean.TRUE.equals(registryConfig.getUseAsMetadataCenter());
    }
    private MetadataReportConfig registryAsMetadataCenter(RegistryConfig registryConfig) {
        String protocol = registryConfig.getProtocol();
        Integer port = registryConfig.getPort();
        String id = "metadata-center-" + protocol + "-" + port;
        MetadataReportConfig metadataReportConfig = new MetadataReportConfig();
        metadataReportConfig.setId(id);
        if (metadataReportConfig.getParameters() == null) {
            metadataReportConfig.setParameters(new HashMap<>());