Normalize converters output

[Reinmar]

When writing a "post-converter" you have a very tricky job to work with the output because there are at least 3 possible types of it – undefined, array or document fragment/element. This forced me to write some odd tests when working on this code:

https://github.com/ckeditor/ckeditor5-paragraph/blob/df99c8d48139bce3186462550e33ffdbf3218cec/src/paragraph.js#L191-L201

I think that you should be always able to expect the same kind of input and output.

[Reinmar]

Seems to be fixed by our conversion refactor a year ago.