[Bug Report] el-time-picker value is null or Invalid Date when is-range is true

### Element UI version
2.13.0

### OS/Browsers version
windows 10/chrome 80.0.3987.149

### Vue version
2.5.2

### Reproduction Link
https://jsfiddle.net/aylan/1zLrth6p/

### Steps to reproduce
el-time-picker value is null or Invalid Date when el-time-picker is-range is true

### What is Expected?
[ Tue Mar 24 2020 10:28:52 GMT 0800 (中国标准时间), Tue Mar 24 2020 10:28:52 GMT 0800 (中国标准时间) ]

### What is actually happening?
[null, null] or [ Invalid Date, Invalid Date ]