fix bug with cost schedules confusing 0 quantity with no quantities

a bit related to #4704

1) fixed util.cost.get_total_quantity
2) fixed similar issue in cost.data that calculates the final value that user will see in UI
3) changed UI, "-" is shown when there are no quantities and "0" is when quantities are there but they just equal to zero.

Before - https://i.imgur.com/EO53DhM.png
After - https://i.imgur.com/H6rK4sP.png

fyi @myoualid

src/blenderbim/blenderbim/bim/module/cost/data.py

@@ -140,7 +140,7 @@ class CostSchedulesData:
                 data["UnitBasisUnitSymbol"] = "U"
             if cost_value.Category == "*":
                 is_sum = True
-        cost_quantity = data["TotalCostQuantity"] or 1
+        cost_quantity = 1 if data["TotalCostQuantity"] is None else data["TotalCostQuantity"]
         if has_unit_basis:
             data["TotalCost"] = data["TotalAppliedValue"] * cost_quantity / data["UnitBasisValueComponent"]
         else:

src/blenderbim/blenderbim/bim/module/cost/ui.py

@@ -634,7 +634,7 @@ class BIM_UL_cost_items_trait:
         layout.label(text=cost_item["UnitBasisUnitSymbol"])
 
     def draw_total_quantity_column(self, layout, cost_item):
-        if cost_item["TotalCostQuantity"]:
+        if cost_item["TotalCostQuantity"] is not None:
             label = "{0:.2f}".format(cost_item["TotalCostQuantity"]) + f" {cost_item['UnitSymbol'] or '-'}"
             layout.label(text=label)
         else:

src/ifcopenshell-python/ifcopenshell/util/cost.py

@@ -40,7 +40,12 @@ def get_primitive_applied_value(applied_value: Union[ifcopenshell.entity_instanc
 def get_total_quantity(root_element: ifcopenshell.entity_instance) -> Union[float, None]:
     # 3 IfcPhysicalQuantity Value
     if root_element.is_a("IfcCostItem"):
-        return sum([q[3] for q in root_element.CostQuantities or []]) or None
+        # Different output for no quantities and zero quantites
+        # as they have different meaning in IFC.
+        quantities = root_element.CostQuantities
+        if not quantities:
+            return None
+        return sum([q[3] for q in quantities])
     elif root_element.is_a("IfcConstructionResource"):
         quantity = root_element.BaseQuantity
         return quantity[3] if quantity else 1.0