[bug report] form validation date rule error

### Element UI version
2.13.2

### OS/Browsers version
win10/ Chrome83.0.4103.116

### Vue version
3.0.0-rc.5

### Reproduction Link
https://codepen.io/bobsli/pen/YzqwNbo?editable=true%3Dhttps%3A%2F%2Felement.eleme.cn%2F2.13%2F

### Steps to reproduce
官方文档的示例代码中
`

`
改为如下：
`

`
然后控制台报错

### What is Expected?
控制台不报错误信息

### What is actually happening?
控制台报错