[Bug Report] 表格同时加上:show-header="false"和height就会报错

### Element UI version
2.12.0

### OS/Browsers version
win10,chrome76.0.3809.100

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/nyu6fcdp/

### Steps to reproduce
table加上
:height="200"
:show-header="false"

就会导致
// fix issue (https://github.com/ElemeFE/element/pull/16956)
var headerTrElm = headerWrapper.querySelector('.el-table__header tr')
这行报错, headerWrapper是undefined

### What is Expected?
不报错

### What is actually happening?
报错, 2.12.0版本才出现的问题