When creating a card, formatted name alone is not accepted

[hydrargyrum]

Following #171 the FN field (formatted name, full name) field is now accepted and it can be generated from N field.
However, when submitting a new card (with khard new) setting only "formatted name" and "email", no "first name", "last name", the card is rejected, see https://github.com/scheibler/khard/blob/master/khard/carddav_object.py#L1031
That card should be accepted, and the FN and emails fields be set.

[lucc]

I think this was originaly a safety mesure to enure that the FN field could be generated. Maybe we can modify the logic in khard to do less checks ourself and use the validation from vobject instead.

[lucc]

PRs welcome.