当只有一种类别数据时，使用图例插件， 图例虽然现实出来了但是不能点击显示隐藏

- [ ] I have searched the [issues](https://github.com/antvis/f2/issues) of this repository and believe that this is not a duplicate.

### Reproduction link
[http://jsrun.net/rtd/iC2Kp](http://jsrun.net/rtd/iC2Kp)

### Steps to reproduce
http://jsrun.net/rtd/iC2Kp

### What is expected?
点击图例显示隐藏

### What is actually happening?
当只有一种类别数据时，使用图例插件， 图例虽然现实出来了但是不能点击显示隐藏

| Environment | Info |
|---|---|
| f2 | 3.6.3 |
| System | win7 |
| Browser | chrome 81.0.4044.138（正式版本） （32 位） |

Start with the reproduction link at http://jsrun.net/rtd/iC2Kp and inspect the legend plugin behavior in F2 3.6.3 when the data has one category. Trace the click handling for the rendered legend, then verify that clicking it toggles the category's visibility without affecting other chart behavior.

Written by the indexing model from the issue text.