Empty header row cells should be TDs

Problem/Motivation

If an empty cell appears inside thead, it should be an attribute-less td element.

See the w3c's Tables with Two Headers example.

Steps to reproduce

1. Create a table with the responsive table tool.
2. Make sure the table has an empty cell in the row header.
3. Save and view the content.
4. Inspect the empty header cell.
5. Verify that it renders a th with scope="col".

Proposed resolution

For empty header row cells, render <td></td> instead of <th scope="col"></th>

Remaining tasks

1. Change the empty cells, as above.

User interface changes

None.

API changes

None.

Data model changes

None.