[Bug Report] Cascader shows scroll on tabbing even if items are not overflowing

### Element UI version
2.15.5

### OS/Browsers version
MacOS

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/liminesredep/pen/abyPqGx

### Steps to reproduce
Go to your Cascader page on Element Ui and tab to the first Cascader. When the Cascader is focused, press your down key on your keyboard. This will open the Cascader. Now you should see that the Cascader displays a vertical/horizontal scrollbar even if the items are not overflowing the container of the cascader.

### What is Expected?
When tabbing to the Cascader and pressing the down key, it should not display a scrollbar if the items are not overflowing the Cascader.

### What is actually happening?
When tabbing to the Cascader and pressing the down key, the component display a scrollbar both horizontally and vertically even if the items are not overflowing the Cascader.