[Bug Report] table里面的树形懒加载，点击第二层的时候，load回调函数里面treeNode.level就是空字符串了

### Element UI version
2.13.2

### OS/Browsers version
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.79 Safari/537.36

### Vue version
3.0.0-rc.10

### Reproduction Link
https://codepen.io/aadelina/pen/LYNOPgR?editable=true%3Dhttps%3A%2F%2Felement.eleme.cn%2F

### Steps to reproduce

export default {
data() {
return {
tableData1: [{
id: 1,
date: '2016-05-02',
name: '王小虎',
address: '上海市普陀区金沙江路 1518 弄'
}, {
id: 2,
date: '2016-05-04',
name: '王小虎',
address: '上海市普陀区金沙江路 1517 弄'
}, {
id: 3,
date: '2016-05-01',
name: '王小虎',
address: '上海市普陀区金沙江路 1519 弄',
hasChildren: true
}, {
id: 4,
date: '2016-05-03',
name: '王小虎',
address: '上海市普陀区金沙江路 1516 弄'
}]
}
},
methods: {
load(tree, treeNode, resolve) {
console.log(treeNode)
if(tree.id === 31){
resolve([
{
id: 41,
date: '2016-05-01',
name: '王小虎',
address: '上海市普陀区金沙江路 1519 弄'
}, {
id: 42,
date: '2016-05-01',
name: '王小虎',
address: '上海市普陀区金沙江路 1519 弄'
}
])

}else{

resolve([
{
id: 31,
date: '2016-05-01',
name: '王小虎',
address: '上海市普陀区金沙江路 1519 弄',
hasChildren:true
}, {
id: 32,
date: '2016-05-01',
name: '王小虎',
address: '上海市普陀区金沙江路 1519 弄'
}
])

}

}
},
}

### What is Expected?
期待点击第二层的时候，打印出来的treeNode.level = 1

### What is actually happening?
实际点击第二层的时候，打印出来的treeNode.level 是空字符串