CDB3-4136 Fix 500 error billing endpoint for procedures with empty classification schema

https://procedure-dev.prozorro.sale/api/procedures/642e7459a9f7fec78597aee4
https://procedure-dev.prozorro.sale/api/billing/accountData/LLE001-UA-20230406-95906

https://procedure-dev.prozorro.sale/api/procedures/642e7569a9f7fec78597aeec
https://procedure-dev.prozorro.sale/api/billing/accountData/LLP001-UA-20230406-44590

https://procedure-dev.prozorro.sale/api/procedures/642e765da9f7fec78597aef6
https://procedure-dev.prozorro.sale/api/billing/accountData/LLD001-UA-20230406-11675

https://procedure-dev.prozorro.sale/api/procedures/642e784da9f7fec78597aefc
https://procedure-dev.prozorro.sale/api/billing/accountData/AAE001-UA-20230406-65502