Combo drill-down back level display problem

### Describe the bug / 问题描述

When the combo is collapsed and all the internal elements are deleted, the combo is opened again and the internal elements are redrawn. It is found that there is a problem with the edge level, which is always 1. However, the combo and the point are not affected.

### Reproduction link / 重现链接

_No response_

### Steps to Reproduce the Bug or Issue / 重现步骤

_No response_

### G6 Version / G6 版本

🆕 5.x

### Operating System / 操作系统

macOS

### Browser / 浏览器

Chrome

### Additional context / 补充说明

_No response_

No reproduction link, steps, files, or tests are provided. Start by reproducing the described sequence in G6 5.x on Chrome: collapse a combo, delete its internal elements, reopen it, and redraw them. Done means the redrawn edges retain the correct level while the combo and point remain unaffected.

Written by the indexing model from the issue text.