[Bug Report] dragging issue in el-tree: draggingNode can not be dropped into an expanded dropNode before draggingNode

### Element UI version
2.14.1

### OS/Browsers version
windows10

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/DeltaNing/pen/bmOyZP

### Steps to reproduce
1. Expand a node（dropNode） that contains some nodes
2. dragging a node(draggingNode) which behind dropNode into dropNode

### What is Expected?
The draggingNode can be dropped into the dropNode while this dropNode'height only contains its self's height

### What is actually happening?
The draggingNode can not be dropped into the dropNode which'height contains childNode's height of the dropNode