[Bug Report] Undefined as class-name in expand column type

[Tenutes]

Element UI version

2.15.0

OS/Browsers version

MacOS / Chrome latest version

Vue version

2.6.12

Reproduction Link

https://codepen.io/pen/?&prefill_data_id=b8883d16-9d2d-441d-9e6e-055a46169d28

Steps to reproduce

Open devtolls. Navigate to expandable column cell.

What is Expected?

No undefined class in column classes

What is actually happening?

classes: el-table_{number}column{number} undefined el-tableexpand-column el-tablecell. This behavior caused by https://github.com/ElemeFE/element/blob/55bac06f0f9e26b820518243f3987cab9699001b/packages/table/src/table-body.js#L210 where is column.className is undefined

[iarmankhan]

reproduction link is broken, but I can confirm this issue. For some reason type="expand" is not doing anything in my setup though

[Tenutes]

Hmm... The reproduction link is from the official docs. https://element.eleme.io/#/en-US/component/table#expandable-row Here you can see that kind of bug too.