[Feature Request] cancels sorting, the sort-change event return null

### Existing Component
是

### Component Name
table-header

### Description
表格只支持单条件排序，或许当用户取消排序时 sort-change 事件可以返回 prop 和 column 可以修复此问题，如下

\element-ui\packages\table\src\table-header.js line 504 505

The table only supports single condition sorting. Perhaps when the user cancels sorting, the sort-change event can return prop and column to fix this problem, as follows