[Bug Report] el-checkbox validate time error

### Element UI version
2.15.7

### OS/Browsers version
macOS / Chrome 94.0.4606.61（正式版本） (x86_64)

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/wrma/pen/mdBJbxV?editors=1010

### Steps to reproduce
1. select a checkbox
2. show error tips
3. clear checkbox
4. error tips disappear

（ps: the input is normal）

### What is Expected?
when checkbox is selected, don't show error tip

### What is actually happening?
show error tip