[Bug Report] 【el-input-number】The value that should not be entered is undefined

### Element UI version
2.14.1

### OS/Browsers version
chrome8.0

### Vue version
2.6.12

### Reproduction Link
https://jsfiddle.net/3sfty1zk/1/

### Steps to reproduce
Clear the content, do not enter any value, and then leave the input box

I am very happy to participate in the open source of element,
but I may need your help, because I am not sure about some of the specifications of our element

### What is Expected?
I expect to display the minimum value by default when there is no content in the input box

### What is actually happening?
When the input box has no content, the value bound to v-model becomes undefined