[Bug Report] Cascader懒加载与multiple或者checkStrictly不兼容

### Element UI version
2.14.1

### OS/Browsers version
windows10/Google Chrome 87.0.4280.66

### Vue version
3.0.3

### Reproduction Link
https://jsfiddle.net/sch2y5ux/

### Steps to reproduce
Cascader设置lazy后，在添加multiple或者checkStrictly

### What is Expected?
leaf为true的，点击时不要再触发lazyLoad。multiple的对勾正确勾选

### What is actually happening?
resolve中的数据leaf为true时，点击仍然会触发lazyLoad，并且回显数据时，multiple的对勾不会勾选