- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] 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: 3.0.0


### Steps to reproduce this issue

1. dubbo config-center 相关yml配置如下
```
dubbo:
  ...
  config-center:
    address: nacos://xxxx:8848
    username: someusername
    password: xxxx
```

2. 在执行 org.apache.dubbo.config.bootstrap.DubboBootstrap#startConfigCenter 方法时会调用 configCenterConfig.refresh(); 方法。
3. setAddress方法，会覆盖已经设置过的username和password

```
public void setAddress(String address) {
        this.address = address;
        if (address != null) {
            try {
                URL url = URL.valueOf(address);
                setUsername(url.getUsername());
                setPassword(url.getPassword());
                updateIdIfAbsent(url.getProtocol());
                updateProtocolIfAbsent(url.getProtocol());
                updatePortIfAbsent(url.getPort());
                updateParameters(url.getParameters());
            } catch (Exception ignored) {
            }
        }
    }
```

### Expected Result

希望设置前先判断是否已经设置过相关属性值



public ConfigCenterConfig() {
    }
    @Override
    protected void checkDefault() {
        super.checkDefault();
        if (namespace == null) {
            namespace = CommonConstants.DUBBO;
        }
        if (group == null) {
            group = CommonConstants.DUBBO;
        }
        if (timeout == null) {
            timeout = 3000L;
        }
        if (check == null) {
            check = true;
        }
        if (configFile == null) {
            configFile = CommonConstants.DEFAULT_DUBBO_PROPERTIES;
        }
    }
    public URL toUrl() {
        Map<String, String> map = new HashMap<>();
        appendParameters(map, this);
        if (StringUtils.isEmpty(address)) {
            address = ANYHOST_VALUE;
        }
        map.put(PATH_KEY, ConfigCenterConfig.class.getSimpleName());
        
        if (StringUtils.isEmpty(map.get(PROTOCOL_KEY))) {