[bug report] no enter active animation in El image preview

### Element UI version
2.13.2

### OS/Browsers version
mac 10.15.5/chrome 83.0.4103.61

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/carlhong/pen/OJMLMzQ

### Steps to reproduce
点击图片即可

### What is Expected?
显示预览大图有enter-active动画，关闭有leave-active动画

### What is actually happening?
显示预览大图时没有enter-active动画，不过关闭时有leave-active动画