- [ ] 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
```

ProviderConfig providerConfig = new ProviderConfig();
                configManager.addProvider(providerConfig);
                providerConfig.refresh();
                return providerConfig;
            });
        }
        for (ProviderConfig providerConfig : configManager.getProviders()) {
            ConfigValidationUtils.validateProviderConfig(providerConfig);
        }
        
        Collection<ConsumerConfig> consumers = configManager.getConsumers();
        if (CollectionUtils.isEmpty(consumers)) {
            configManager.getDefaultConsumer().orElseGet(() -> {
                ConsumerConfig consumerConfig = new ConsumerConfig();
                configManager.addConsumer(consumerConfig);
                consumerConfig.refresh();
                return consumerConfig;
            });
        }
        for (ConsumerConfig consumerConfig : configManager.getConsumers()) {
            ConfigValidationUtils.validateConsumerConfig(consumerConfig);
        }
        
        ConfigValidationUtils.validateMonitorConfig(getMonitor());
        
        ConfigValidationUtils.validateMetricsConfig(getMetrics());
        
        ConfigValidationUtils.validateModuleConfig(getModule());
        
        ConfigValidationUtils.validateSslConfig(getSsl());