[Bug Report] Date Picker does not format date on tab blur

### Element UI version
2.13.0

### OS/Browsers version
Windows 10, Chrome Version 79.0.3945.130

### Vue version
2.6.11

### Reproduction Link
https://jsfiddle.net/gsrezfuo/4/

### Steps to reproduce
- focus date picker
- type date in format of "MMddyyyy"
- press tab key

### What is Expected?
The date provided in format MMddyyyy gets formatted to MM/dd/yyyy and the value is emitted .

### What is actually happening?
After blur the datepickers input box is cleared and the mound value is null. The date no longer gets formatted into date string "MM/dd/yyyy" after pressing tab. Upon updating from 2.8.2 to 2.13.0 this functionally is now gone.