🐛 [BUG] g2plot 2.x 箱型图组件 异常值分布及值标签显示问题

### 🐛 Bug description [Please make everyone to understand it]
1. 异常值问题，当只有一条或两条数据中有异常值 outliers配置时，异常值分布位置异常
2. 值标签不管现实谁，都在异常值上方
3. 复现地址：https://codesandbox.io/s/serene-thompson-r09m4?file=/index.ts:142-150


* **G2Plot Version**: 2.3.39
* **Platform**: g2plot 2.x

Start with the reproduction in the linked CodeSandbox, especially index.ts around lines 142-150, and verify the outlier positions and value-label placement for one- and two-point datasets. Compare the observed box plot against the expected distribution and label location; done means both behaviors are corrected for the reported configuration and the reproduction no longer shows the issue.

Written by the indexing model from the issue text.