[Content Editor] Changes between draft and (un)published variants are not displayed

Short description

After the Keep draft functionality was introduced in 14.2, the changes between the draft and the published/unpublished version are not shown in the preview. This will impede other users who want to edit the draft, to see what has been changed.

Steps to reproduce

1. Go to Content perspective
2. Open a document
3. Make some changes
4. Select Keep draft option

Expected result:

The changes from step 3 are shown in the preview.

Actual result:

Nothing is shown.