[Bug Report] row-id为0时候bug

### Element UI version
2.15.7

### OS/Browsers version
win10 chrome

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/zhengfeiwen/pen/LYzMXNQ

### Steps to reproduce
配置table的row-id一个数据id为0:number，将无法展开。
在packages/table/src/store/tree.js
toggleTreeExpansion(row, expanded) {
this.assertRowKey();

const { rowKey, treeData } = this.states;
const id = getRowIdentity(row, rowKey);
const data = id && treeData[id]; // 这边判断是不是可以修改一下，id为0情况判断为false了。
if (id && data && ('expanded' in data)) {
const oldExpanded = data.expanded;
expanded = typeof expanded === 'undefined' ? !data.expanded : expanded;
treeData[id].expanded = expanded;
if (oldExpanded !== expanded) {
this.table.$emit('expand-change', row, expanded);
}
this.updateTableScrollY();
}
}

### What is Expected?
点击展开， children tree应该被展开

### What is actually happening?
没有反应