- [ ] 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: latest
* Operating System version: xxx
* Java version: xxx

### Steps to reproduce this issue

1. Config a Monitor and specify the value of its protocol to registry(address is null)
2. run the dubbo application

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

### Expected Result
Monitor effect and no warning log about monitor

What do you expected from the above steps？

### Actual Result
The log says that There's no valid monitor config found
What actually happens?
the code of The logic of org.apache.dubbo.config.MonitorConfig#isValid is

```
public boolean isValid() {
    return StringUtils.isNotEmpty(address);
}
```
actually, if not specify address but specify the monitor config's protocol to registry, that is OK.

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

```
[20/08/19 14:22:59:133 CST] main  INFO config.AbstractConfig:  [DUBBO] There's no valid monitor config found, if you want to open monitor statistics for Dubbo, please make sure your monitor is configured properly., dubbo version: , current host: 192.168.99.1

```

}
    public void setVersion(String version) {
        this.version = version;
    }
    public Map<String, String> getParameters() {
        return parameters;
    }
    public void setParameters(Map<String, String> parameters) {
        checkParameterName(parameters);
        this.parameters = parameters;
    }
    public Boolean isDefault() {
        return isDefault;
    }
    public void setDefault(Boolean isDefault) {
        this.isDefault = isDefault;
    }
    public String getInterval() {
        return interval;
    }
    public void setInterval(String interval) {
        this.interval = interval;
    }
    @Override
    @Parameter(excluded = true)
    public boolean isValid() {
        return StringUtils.isNotEmpty(address);
    }
}