[Page edit] Can't remove a page's preview image

**Describe the bug**
Once a preview image is added to a page, the page can not be left with no image.

**To Reproduce**
Steps to reproduce the behavior:
1. Log in to the admin and go to create/edit a page
2. Add a preview image to the page (click on Attach) & save the page
3. Remove the preview image (click on the bin) & save the page
4. Refresh the page
5. Observe that the image is still there

**Expected behavior**
Once a page's preview image has been deleted, the image should not appear after a refresh.

**Screenshots**

**Additional context**
When the page is shared, the preview appears there.