[Accessibility] Empty table headers should use <td> instead of <th>

[afgomez]

As part of my work for elastic/kibana#37539 I saw that EuiBasicTable sometimes renders empty table headers, for example, in headers for row expanders. The WAVE tool considers this an error, and guidelines to fix it.

If the table cell is a header, provide text within the cell that describes the column or row. If the cell is not a header or must remain empty (such as the top-left cell in a data table), make the cell a rather than a .

How to reproduce

1. Install the WAVE browser extension
2. Go to https://elastic.github.io/eui/#/tabular-content/tables
3. Run the tool. The errors appear

Suggested solution

When the component knows that a table header will be empty because the column is used for non-content (i.e. row expanders), use a <td> instead.

[cchaos]

Haha, thanks @afgomez Yes we also have an existing ticket for this one as well #840. We are in the middle of an a11y push for some components (slow-going, but going) and we've got this on our meta list #2262. I'm mainly closing this one since we don't like to have duplicates and we also agree with your solution.