[bug report] datetimepicker date time selector, select thick and don't click OK, the result will still return to the drop-down box

### Element UI version
2.15.0

### OS/Browsers version
windows10/goole

### Vue version
2.6.12

### Reproduction Link
https://github.com/ElemeFE/element/blob/dev/FAQ.md#%E5%B8%B8%E8%A7%81%E9%97%AE%E9%A2%98

### Steps to reproduce
API网站el-date-picker datetime类型下，选中日期后点击空白处，选择结果仍应用于input框

### What is Expected?
选中日期后点击空白处收回日期选择面板，当前input框展示为上一次确认的选中日期

### What is actually happening?
选中日期后点击空白处，选择结果仍应用于input框