[bug report] the default time set by the El date picker component detault time = "00:00:00" does not take effect

### Element UI version
2.15.6

### OS/Browsers version
Windows 10 家庭中文版 / chrome(83)

### Vue version
2.6.14

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

### Steps to reproduce
在默认 el-date-picker 组件加上这两个属性 detault-time="00:00:00" value-format="yyyy-MM-dd hh:mm:ss" 如下：

### What is Expected?
期待时间是：00:00:00

### What is actually happening?
实际时间是：12:00:00