[Bug Report] el-select当下拉的数据中有value是number类型的0时，选择返回的是undefined

### Element UI version
2.15.13

### OS/Browsers version
win10/谷歌浏览器

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/zouqinchuan/pen/KKrBaYZ

### Steps to reproduce
options: [
{ value: 1, label: "是" },
{ value: 0, label: "否" },
],

### What is Expected?
能够准确的返回value

### What is actually happening?
选择number类型的0数据时，返回的undefined