Update subitem_number assignment in calculations.py

- Changed the assignment of subitem_number from None to 0 in the apply_calculations_after_values function to ensure consistent initialization of subitem data during calculations.

backend/api/v1/modules/a76/items/imports/validators/calculations.py

@@ -25,7 +25,7 @@ def apply_calculations_after_values(
     """
     fa_data = getattr(line, "fa_data", None)
     if fa_data is not None and not getattr(fa_data, "is_subitem", True):
-        fa_data.subitem_number = None
+        fa_data.subitem_number = 0
 
     invoice_date = db.query(InvoiceHeader.invoice_date).filter(
         InvoiceHeader.id == line.invoice_id,