Value error when [bug report] uses [autocomplete] to clear the input content

### Element UI version
2.13.1

### OS/Browsers version
70.0.3538.102

### Vue version
3.0.0-beta.9

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

### Steps to reproduce
https://codepen.io/rsenhuang/pen/JjYMgOx?editable=true%3Dhttps%3A%2F%2Felement.eleme.cn%2F

### What is Expected?
there should not have any error when you clear what you inputed in autocomple

### What is actually happening?
throw error: Cannot read property 'value' of undefined, which causes by inner input module emit change event with empty string but the autocomple module treats arguments[0] as an Object like 'event.target.value'.