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

### Environment

* Dubbo version: xxx
* Operating System version: xxx
* Java version: xxx

### Steps to reproduce this issue

nacos registry serviceName may conflict when we have two provider with same service but one config version as A and left group empty and
one config group as A and left version empty

the two provider would get registered with same key    provdier:serviceName:A.

the cause of this issue is we ignore empty value when we construct the key,  relative code is 

```java
 private String getServiceName(URL url) {
        String category = url.getParameter(CATEGORY_KEY, DEFAULT_CATEGORY);
        return getServiceName(url, category);
    }

    private String getServiceName(URL url, String category) {
        StringBuilder serviceNameBuilder = new StringBuilder(category);
        append(serviceNameBuilder, url, INTERFACE_KEY);
        append(serviceNameBuilder, url, VERSION_KEY);
        append(serviceNameBuilder, url, GROUP_KEY);
        return serviceNameBuilder.toString();
    }

private void append(StringBuilder target, URL url, String parameterName) {
        target.append(SERVICE_NAME_SEPARATOR);
        String parameterValue = url.getParameter(parameterName);
        if (!StringUtils.isBlank(parameterValue)) {
            target.append(parameterValue);
        }
    }
``` 

i think we should just keep a SEPARATOR as placeholder for empty value.


1. xxx
2. xxx
3. xxx

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

### Expected Result

What do you expected from the above steps？

### Actual Result

What actually happens?

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

```
Just put your stack trace here!
```

collection.removeIf(data -> !filter.accept(data));
    }
    private List<String> doGetServiceNames(URL url) {
        String[] categories = getCategories(url);
        List<String> serviceNames = new ArrayList<>(categories.length);
        for (String category : categories) {
            final String serviceName = getServiceName(url, category);
            serviceNames.add(serviceName);
        }
        return serviceNames;
    }
    private List<URL> buildURLs(URL consumerURL, Collection<Instance> instances) {
        if (instances.isEmpty()) {
            return Collections.emptyList();
        }
        List<URL> urls = new LinkedList<>();
        for (Instance instance : instances) {
            URL url = buildURL(instance);
            if (UrlUtils.isMatch(consumerURL, url)) {
                urls.add(url);
            }
        }
        return urls;
    }
    private void subscribeEventListener(String serviceName, final URL url, final NotifyListener listener)
            throws NacosException {
        if (!nacosListeners.containsKey(serviceName)) {
            EventListener eventListener = event -> {
                if (event instanceof NamingEvent) {