- [ ] 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!
```

String[] segments = StringUtils.split(serviceName, SERVICE_NAME_SEPARATOR);
            int length = segments.length;
            if (length < 3) { 
                return false;
            }
            String category = segments[CATEGORY_INDEX];
            if (!ArrayUtils.contains(categories, category)) { 
                return false;
            }
            String serviceInterface = segments[SERVICE_INTERFACE_INDEX];
            if (!WILDCARD.equals(targetServiceInterface) &&
                    !StringUtils.equals(targetServiceInterface, serviceInterface)) { 
                return false;
            }
            String version = segments[SERVICE_VERSION_INDEX];
            if (!WILDCARD.equals(targetVersion) &&
                    !StringUtils.equals(targetVersion, version)) { 
                return false;
            }
            String group = length > 3 ? segments[SERVICE_GROUP_INDEX] : null;
            
            return group == null || WILDCARD.equals(targetGroup)
                    || StringUtils.equals(targetGroup, group);
        });
    }
    private <T> void filterData(Collection<T> collection, NacosDataFilter<T> filter) {