APIv2:  Add `default`s and `allow_null=True` to some serializer fields
APIv2:  Add `default`s and `allow_null=True` to some serializer fields

Add `default`s and `allow_null=True` to serializer fields where the
corresponding model field has the same properties.

OWASP/django-DefectDojo,OWASP/django-DefectDojo,rackerlabs/django-DefectDojo,OWASP/django-DefectDojo,rackerlabs/django-DefectDojo,rackerlabs/django-DefectDojo,OWASP/django-DefectDojo,rackerlabs/django-DefectDojo,OWASP/django-DefectDojo