- [ ] 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
```

ShutdownHookCallbacks.INSTANCE.addCallback(new ShutdownHookCallback() {
            @Override
            public void callback() throws Throwable {
                DubboBootstrap.this.destroy();
            }
        });
    }
    public void unRegisterShutdownHook() {
        DubboShutdownHook.getDubboShutdownHook().unregister();
    }
    private boolean isOnlyRegisterProvider() {
        Boolean registerConsumer = getApplication().getRegisterConsumer();
        return registerConsumer == null || !registerConsumer;
    }
    private String getMetadataType() {
        String type = getApplication().getMetadataType();
        if (StringUtils.isEmpty(type)) {
            type = DEFAULT_METADATA_STORAGE_TYPE;
        }
        return type;
    }
    public DubboBootstrap metadataReport(MetadataReportConfig metadataReportConfig) {
        configManager.addMetadataReport(metadataReportConfig);
        return this;
    }
    public DubboBootstrap metadataReports(List<MetadataReportConfig> metadataReportConfigs) {
        if (CollectionUtils.isEmpty(metadataReportConfigs)) {
            return this;
        }
        configManager.addMetadataReports(metadataReportConfigs);