[Bug Report] el-autocomplete clearable can not trigger suggestion callback

### Element UI version
2.11.1

### OS/Browsers version
macOS 10.13 / chrome 79

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/zhidanwang19910111/pen/GRJmBwg

### Steps to reproduce
1. el-autocomplete input '三', and then click clear icon
2. then auto focus on input and input '三' again

### What is Expected?
after clicking clear icon, auto focus input, and inputing something can show suggestion list

### What is actually happening?
suggestion hide,can not show again, unless blur ,and focus again