KRACOEUS-8584:  creating a modular budget doesn't create the period modular budget

Up to you. With the relationship as it is it can't be persisted on the initial save because there isn't a budgetPeriodId. But if it were referencing the budgetPeriod DO instead, it could be persisted initially. And referencing budgetPeriod instead of the id is what we've been trying to do, but I understand if you don't want to mess with it now.