[Bug Report] Form validation blocks submit with form-item bound to child object of form

### Element UI version
2.15.0

### OS/Browsers version
Chrome

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/aldeng11/pen/MWjRXaM

### Steps to reproduce
Click submit button in codepen.
Required validation fails.
Enter text.
Click submit.

### What is Expected?
The form submits. Required section has been made.

### What is actually happening?
Form submits with failed validation. Form still shows invalid. If you bind el-input to ruleForm.name, it works. It fails if el-input bound to ruleForm.schema.name.