Improve empty cells accessibility

Problem/Motivation

Empty tables cells (td) aren't announced to screen readers and in some cases it can make the screen reader to stop reading the table completely.

Steps to reproduce

Create a table with empty cells and try navigating it with a screen reader app.

Proposed resolution

Add `

` to empty cells. This could either be done on the CKeditor plugin or on the Tabled plugin.

Remaining tasks

User interface changes

API changes

Data model changes