Inserting image into empty editor prevents adding other content

[eahrold]

Is this a bug report or feature request? (choose one)

🐞 Bug report

💻 Version of CKEditor

Current version on ckeditor5 docs page (most likely beta-1)

📋 Steps to reproduce

1. go to https://docs.ckeditor.com/ckeditor5/latest/features/image.html
2. Choose the first CKEditor instance
3. Remove all content from editor
4. Add an image
5. Try to add text content to the editor

✅ Expected result

Either pressing arrow keys, or clicking off image, will deselect image and allow adding content before / after.

❎ Actual result

Image consumes entire width of editor, and will not release focus. If the image is the only element in the editor, cannot click off to add text before/after

[Reinmar]

That's a DUP of #407.