[bug report] when all the lines in the list are disabled, the check box in the header should also be disabled

### Element UI version
2.13.2

### OS/Browsers version
win10/chrome

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/gh-xiaoxiao/pen/QWyjZVr

### Steps to reproduce
当列表中的所有行都被禁选时

### What is Expected?
头部的全选复选框应为禁用状态

### What is actually happening?
头部的全选复选框依然可以点击