🤔 [QUESTION] 分组图表的图例legend排序问题

### 🐛 Question description [Please make everyone to understand it]

分组柱状图第一组数据不全，legend怎么按图片中的红字升降排序
### 💻 Link to minimal reproduction

Please provide a link by forking these links [g2plot](https://codesandbox.io/s/g2plot-reproduction-template-gyej1) or GitHub repo. What is a minimal reproduction, and why is it required?

### 🏞 Expected result

### 🚑 Any additional [like screenshots]

* **G2Plot Version**:
^2.3.40

* **Platform**:

Reproduce the report with G2Plot ^2.3.40 using a grouped bar chart where the first data group is incomplete, referring to the attached screenshot. The issue has no minimal reproduction or expected-result details, so first determine the current legend ordering and the intended ascending or descending ordering before identifying what would count as resolved.

Written by the indexing model from the issue text.