Restored a part of code and added a test for it

[pomek]

Suggested merge commit message (convention)

Tests: Restored a condition that caused an error in test logs in the @ckeditor/ckeditor5-image package. Added a test to cover the code and make 100% CC again. Closes ckeditor/ckeditor5#5892.

[coveralls]

Coverage remained the same at 100.0% when pulling 2e29e9a on i/5892 into 64209ec on master.

[Reinmar]

You shouldn't just copy an image test to this package. Image is a consumer of this package and has its own tests. This package should has its own, generic tests. You should check how the image test triggers this single case and transform this to a generic test for this package. One which will not create dependencies on this package's consumer (image became this package's dependency and this is already fishy).

[Reinmar]

As commented.

[pomek]

Because CI does not work for a while, I'm pasting screenshots from CC report:

@Reinmar, ready for review once again.

[Reinmar]

I closed this as a "Fix" because it actually resolves a bug that we introduced by removing this condition. This bug was manifested in tests only, but it may also perhaps somehow appear irl.