BUG: add col<k> class identifier to blank index name row cells

[attack68]

• closes BUG: Style index name row does not label column class #39716
• tests passed
• linting and typing checks

[jreback]

practically unless a user is setting table styles this doesn't change the display right?

[attack68]

@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.

[jreback]

thanks @attack68