[Bug Report] Select Option Selected Item Problem

### Element UI version
2.4.11

### OS/Browsers version
Browser

### Vue version
2.6.12

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

### Steps to reproduce
1. choose the select option between left side and right side and u will see the dif

### What is Expected?
without value the select option work

### What is actually happening?
when select any item in select option, the select option show all item selected, but the model value is correct
if the options is come with the value data, then this problem will not happened... please fix ...