BUG: add `col<k>` class identifier to blank index name row cells
- [x] closes #39716
- [x] tests passed
- [x] linting and typing checks
practically unless a user is setting table styles this doesn't change the display right? @jreback no this doesn't change user display. 
*Unless* the user has set a table style with some version of `.col` as css selector. In this case that selector will now apply to all cells in the columns *instead of* all cells in the column except one of multiple header rows. thanks @attack68 