[Bug Report] Hidden header bugs

### Element UI version
2.13.1

### OS/Browsers version
window10/Google Chrome 81.0.4044.92

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/pen/?&editable=true=https%3A%2F%2Felement.eleme.cn%2F

### Steps to reproduce
First step ： show-header = 'false'.
second step ： height = '400'

Console Output
Cannot read property 'querySelector' of undefined
at TableLayout.updateElsHeight (table.js:2272)

### What is Expected?
I want to hide the header and set the height to show the scroll bar

### What is actually happening?