Revert "adding paragraph to empty editables"

It was quickly discovered that this change altered the functionality of the
autoparagraph plugin in a way that is contrary to how it is intended to work:
to automatically wrap the immediate children of empty editables in paragraph.
So empty editables should not have paragraphs automatically added to them.

This reverts commit 4fa2a3a521b3c48e74a253f1c1187085d4111cba.