[Bug Report] Numeric input box encountered a bug during form validation

### Element UI version
2.14.1

### OS/Browsers version
Win10/chrome 87.0.4280.141

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/fsqxxx/pen/poEQBvX

### Steps to reproduce
Turn off the required options in the number input box, and delete all numbers after entering the number. At this time, the input box is empty, but the input character type is wrong and the verification fails.

### What is Expected?
Turn off the required options in the number input box, enter the numbers and then delete all numbers, hoping to close the verification and pass the verification

### What is actually happening?
Turn off the required options in the number input box, enter the numbers and then delete all numbers, hoping to close the verification and pass the verification