Previously empty strings where not properly handled and the previous
value persisted.

Is now done in loadingMixin of createOrPatchMixin.

Disabled grays the text - readonly does not.
But readonly still allows to select the textfield.

This overwrites the handleUpdateAfterCreateOrPatch method of the
createOrPatchMixin and adapts it to work with documentations that
where send by the backend as either already created or dummy.