[Bug Report] el-autocomplete issue

### Element UI version
2.15.6

### OS/Browsers version
window 10/chrome

### Vue version
2.6.14

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

### Steps to reproduce
When I add the clear attribute to the "el-autocomplete" component, I enter the keyword to search, click the clear button on the right, and enter the keyword again to search, the drop-down box does not appear.

### What is Expected?
When I click the clear button on the right and enter the keyword again, the drop-down prompt box can appear, instead of the drop-down prompt box only after the focus is lost.

### What is actually happening?
When I click the clear button on the right and enter the keyword again, the drop-down prompt box does not appear.