apps.core.models.assignment: Fixed clean when parentnode is not set.
apps.core.models.assignment: Fixed clean when parentnode is not set.

It used to check for parentnode, however that raises
Period.DoesNotExist. Now we check for parentnode_id instead.

vegarang/devilry-django,devilry/devilry-django,vegarang/devilry-django,devilry/devilry-django,devilry/devilry-django,devilry/devilry-django