Add default values for line item serializers
Add default values for line item serializers

- Once https://github.com/inventree/InvenTree/issues/1687 is fixed, remove default values

inventree/InvenTree,SchrodingersGat/InvenTree,SchrodingersGat/InvenTree,inventree/InvenTree,inventree/InvenTree,SchrodingersGat/InvenTree,SchrodingersGat/InvenTree,inventree/InvenTree