[Bug Report] the result will not change after clearing the daterange of date-picker and clicking search button

### Element UI version
2.13.1

### OS/Browsers version
macOS Big Sur 11.5.2 / Chrome 93.0.4577.82

### Vue version
2.5.17

### Reproduction Link
https://codepen.io/wonz5130/pen/KKqEMjX

### Steps to reproduce
1. select the daterange including start time and end time
2. click search button
3. clear the daterange
4. click search button

### What is Expected?
can show the correct result

### What is actually happening?
still show the previous result