[Backport CMS7-8660 to 7.9] Browser refresh of draft document saves it inadvertently

Create a new document in the CMS, the editor opens in draft mode. Now refresh the browser: the document is shown in preview mode. Next to the draft version an unpublished version is made.
A document that has required fields is saved without any values in these fields. Also the discard option does not work anymore.
This must be prevented since it breaks the workflow.