Fix(Invoicing) Fix crashes when prices are 0 (#3744)(patch)

## Description
AZ reported that samples with the price 0 crashes the entire invoicing flow

### Changed

- Return 0 instead of None if price is 0

Author: Vince-janv

cg/meta/invoice.py

@@ -137,9 +137,7 @@ def _discount_price(self, record: Sample or Pool, discount: int = 0) -> int | No
         priority = self.get_priority(record, for_discount_price=True)
         full_price = getattr(record.application_version, f"price_{priority}")
         discount_factor = 1 - discount / 100
-        if not full_price:
-            return None
-        return round(full_price * discount_factor)
+        return round(full_price * discount_factor) if full_price else 0
 
     def _cost_center_split_factor(
         self, price: int, cost_center: str, percent_kth: int, tag: str, version: str
@@ -162,15 +160,14 @@ def _cost_center_split_factor(
             self.log.append(
                 f"Could not get price for samples with application tag/version: {tag}/{version}."
             )
-            return None
+            return 0
         return split_price
 
     def get_invoice_entity_record(
         self, cost_center: str, discount: int, record: Sample or Pool
     ) -> InvoiceInfo:
         """Return invoice information for a specific sample or pool."""
         application = self.get_application(record=record, discount=discount)
-
         split_discounted_price = self._cost_center_split_factor(
             price=application.discounted_price,
             cost_center=cost_center,