calculate_cost_item_resource_value not to skip 0 costs/quantities

That way all assigned resources will be added as cost values to cost items and it will be more apparent what resources were used for the last calculation and what costs/quantities they had at the time.

src/ifcopenshell-python/ifcopenshell/api/cost/calculate_cost_item_resource_value.py

@@ -107,7 +107,7 @@ def calculate_cost_item_resource_value(file: ifcopenshell.file, cost_item: ifcop
             assert parent_cost
             cost, unit = parent_cost
         quantity = ifcopenshell.util.resource.get_quantity(resource)
-        if not cost or not quantity:
+        if cost is None or quantity is None:
             continue
         if unit and "day" in unit:
             quantity = quantity / 8  # Assume 8 hour working day - TODO implement resource calendar