dubbo-admin-0.6.0：当dubbo服务的注册模式为应用级时，dubbo-admin查询不到服务

- [√] I have searched the [issues](https://github.com/apache/dubbo-admin/issues) of this repository and believe that this is not a duplicate.
- [√] I have checked the [REAMDE](https://github.com/apache/dubbo-admin/blob/develop/README.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo Admin version: 0.6.0
* Registry center detail (type and version): nacos-2.2.4
* Operating System version: linux
* Java version: 1.7

### Steps to reproduce this issue
`dubbo.properties`配置：
```
# instance / interface / all
dubbo.registry.register-mode=instance
```

1. 当 register-mode=instance 时，dubbo-admin 查询不到服务
2. 当 register-mode=all 或 interface 时，dubbo-admin 可以查询到服务

### Expected Result

dubbo.registry.register-mode 为任何值都可以查询到服务。

### Actual Result

当 register-mode=instance 时，dubbo-admin 查询不到服务

No source file or test is named. First reproduce the service-discovery query with dubbo.registry.register-mode set to instance, then compare it with all and interface while checking the Nacos 2.2.4 registry path. Done means dubbo-admin can find services consistently for all three register modes.

Written by the indexing model from the issue text.