[bug report] when there are disabled nodes in El tree, select all (top node) to select the disabled node, but select its child node to select the disabled node;

### Element UI version
2.15.3

### OS/Browsers version
win10 版本20H2/chrome 版本 91.0.4472.124（正式版本） （64 位）

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/qd-pangyongkang/pen/eYWZvVX?editors=1111

### Steps to reproduce
当二级存在禁用时，选择第一级全选，无法选中二级禁用的选项，但是选择其全部三级子项，可以选中二级，并且此时操作全选，无法控制该三级子项

When the second level is disabled, select the first level to be all selected, and the second-level disabled option cannot be selected, but select all the third-level subitems, and the second level can be selected, and the operation is all selected at this time, and the third-level subitems cannot be controlled

### What is Expected?
选择全选时，将树所有节点选中
When selecting All, select all nodes of the tree

### What is actually happening?
选择全选时，存在“非禁用状态的子级”的父级无法被选中
When you select all, a parent with a "child in a non-disabled state" cannot be selected