- [ ] 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

```

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.dubbo.config;
import org.apache.dubbo.common.utils.StringUtils;
import org.apache.dubbo.config.support.Parameter;
import java.util.Map;
/**
 * MonitorConfig
 *
 * @export
 */
public class MonitorConfig extends AbstractConfig {