[Bug Report] el-table enable Tree data and lazy mode,When Tree level is 1 or bigger ,the level value is ''

### Element UI version
2.15.7

### OS/Browsers version
win11/Edge 99.0.1150.46

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/donga-gao/pen/YzYNJLm

### Steps to reproduce
expand 2 more level

### What is Expected?
parent.treeNode.level should be the level of the parent node

### What is actually happening?
parent.treeNode.level is empty str