[Bug Report] DateTimePicker default-time when start time is greater than end time, date select same day, Change the start time, the end time is not optional

### Element UI version
2.15.9

### OS/Browsers version
mac os and windows

### Vue version
2.7.8

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

### Steps to reproduce
1. default-time set start time greater than end time
2. date select same day
3. modify start time
4. End time selection can only select the time after the initial start time, the reality is that the start time has changed
5. Select any date, and those whose end time is less than the initial set start time cannot be selected

### What is Expected?
fix

### What is actually happening?
When the default time set for the second time is less than the first time, after clicking on the date once on the same day, the date is selected thereafter and the end time selection is affected. The example on the official website is problematic