Autoparagraphing ignores empty elements

[Reinmar]

Currently, loading a single image or foo<img>bar into the editor will not trigger the autoparagraphing of that image (if images were treated as inline elements).

This is a conscious decision made in order to simplify #3291, because I had enough fun implementing the behaviour for the text. Also, https://github.com/ckeditor/ckeditor5-paragraph/issues/10 is more actually more important than this issue, so I'd actually spend time fixing that one.

What makes this particular issue even less important is that right now we have only one such element – image, and it's block oriented. Plus, it will have to have its own converter for inline images: https://github.com/ckeditor/ckeditor5-image/issues/8.

[scofalik]

I tried to fix it in naive way, by allowing autoparagraphText converter to be fired also for elements, but this fails because first default to-document-fragment converter is fired.

We should think whether it makes more sense to have autoparagraphText fired before default to-document-fragment converter. For me it makes sense because in autoparagraphText we are trying to fire non-default conversion.

[scofalik]

I tried to fix it in naive way, by allowing autoparagraphText converter to be fired also for elements

Okay, this idea is bad because we should also check whether given element is allowed in paragraph (current autoparagraphText silently assumes that bare text is always allowed in paragraph). To check this, we need to know to which element current element will be converted because we need that info for schema.check(). ATM we are unable to get it because we don't know to what element data.input is supposed to be converted.