Changes ChildB test model to be nullable. This exposes the issue with the dotted notation prefetch not working. After doing this the tests associated with this serializer fail.
Adds additional functionality to allow dotted notation prefetch to work as expected
Updates prefetch so that select_related and prefetch_related are only called if there are relations to prefetch. Avoid the situation where a serializer that does not need the relations accidentally gets them all prefetched.
As per https://github.com/GeeWee/django-auto-prefetching/issues/27