[bug report] unable to modify the tooltip of menu stowed state

### Element UI version
2.15.1

### OS/Browsers version
win10 谷歌浏览器

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/freezyh/pen/abBrPJv

### Steps to reproduce
菜单收起，鼠标移入没有子级的菜单，tooltip默认是dark，不能修改其样式，也没有配置一级菜单的popver-class

### What is Expected?
可以修改或者配置一级菜单收起状态的tooltip

### What is actually happening?
默认是dark