- [ ] 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

```

/**
     * The monitor user name
     */
    private String username;
    /**
     * The password
     */
    private String password;
    private String group;
    private String version;
    private String interval;
    /**
     * customized parameters
     */
    private Map<String, String> parameters;
    /**
     * If it's default
     */
    private Boolean isDefault;
    public MonitorConfig() {
    }
    public MonitorConfig(String address) {
        this.address = address;
    }
    @Parameter(excluded = true)
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;