Empty fields leave bogus empty paragraph as content

[podarcis]

WYSIWYG fields that are empty (have no editorial content) are not persisted with NULL or empty string, but with an empty paragraph <p></p>. This complicates tests in templates or business logic because one does not only need to test for empty, but also for the possible value <p></p>.

For the TinyMCE implementation there has been a (temporary) fix that might relate to this issue: pimcore/admin-ui-classic-bundle#304

Steps to reproduce

1. Create a class with a WYSIWYG and an DataObject instance.
2. Edit the field, but finally empty its content (delete everything in it).
3. Confirm in database, that the field is not stored as NULL or '', but with <p></p>.