[Bug Report] Form resetFields out of expectation

### Element UI version
2.15.6

### OS/Browsers version
93

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/preflower/pen/dyRwzEV

### Steps to reproduce
1. input datepicker and select value
2. click reset button to clear form

### What is Expected?
after resetFields be triggered, conditions.date return undefined

### What is actually happening?
conditions.date return [undefined]