Render block filler in empty lines in container

[pomek]

Suggested merge commit message (convention)

Fix: Block filler will be inserted into the container if its last child is the
element. Closes ckeditor/ckeditor5#4351.

[Reinmar]

👍 Great tests. All works fine for me.

One missing thing will be adding integration tests for setData() and getData(). But this should be done in @ckeditor/ckeditor5-enter.

[coveralls]

Coverage remained the same at 100.0% when pulling a0c8bf0 on t/1422 into 62a0324 on master.