[Bug Report] change label el-tabs__ active-bar width calculation is incorrect

### Element UI version
2.13.1

### OS/Browsers version
Chrome 83.0.4103.61(正式版本)

### Vue version
2.6.10

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

### Steps to reproduce
When el-tab-pane is no content in it, dynamically change the length of the label content

### What is Expected?
When the length of label content changes, el-tabs__active-bar width not change

### What is actually happening?
When the length of label content changes, el-tabs__active-bar width should also change