[Bug Report] Cascader-Style is not cleared when component options are cleared

### Element UI version
2.13.0

### OS/Browsers version
mac

### Vue version
3.0.0-alpha.1

### Reproduction Link
https://codepen.io/pen/?&editable=true

### Steps to reproduce
Cascader component use 'clearable' Just clear the style of the last option;

### What is Expected?
The style returns to its original state

### What is actually happening?
Just clear the style of the last option