[Bug Report] el-select clears value on blur

### Element UI version
2.13.0

### OS/Browsers version
Mac latest

### Vue version
2.6.11

### Reproduction Link
https://element.eleme.io/#/en-US/component/select#remote-search


### Steps to reproduce
1. Focus in the input
1. Type a search
1. Blur the input
1. Focus again

### What is Expected?
1. The input should keep its value on blur
1. On focus again, the suggestion list should be hidden if the input is empty

### What is actually happening?
1. The input is cleared on blur
1. If the value is cleared, the next focus should not show the previous suggestions