[ BUG ] If there is no username configured, a message with ‘@’ will activate the AI robot process.

## Description

 If there is no username configured, a message with ‘@’ will activate the AI robot process.

## Screenshots

<img width="400" alt="image" src="https://github.com/niracler/nyaruko-telegram-bot/assets/24842631/b1e0507d-111c-43b2-9448-fcc377776958">

## Plugin Design

```toml
plugin = [ 'xxx', 'yyy', 'zzz' ]
```