[Bug Report] 表格组件在数据为0时，滚动条在冻结列层级下，无法点击滚动条滚动

### Element UI version
2.14.1

### OS/Browsers version
win10/Google Chrome 86.0.4240.183

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/wf1205952124/pen/WNGNrEK

### Steps to reproduce
点击冻结列下方的滚动条，无法点击滚动

### What is Expected?
冻结列下方的滚动条可以正常点击滚动

### What is actually happening?
点击冻结列下方的滚动条，无法点击滚动