builder: stock optimisation, not all resource might need to be optimized

before, the code was going through an error when no date was found for
doing stock optimisation. But we do date is found, this only means that
the stock is not going below the expected value, so we do not need to do
any optimisation

product/ERP5/mixin/builder.py

@@ -228,17 +228,17 @@ class BuilderMixin(XMLObject, Amount, Predicate):
                                variation_text=movement.getVariationText(),
                                from_date=DateTime(),
                                **kw)
-          assert stop_date is not None
-          max_delay = resource.getMaxDelay(0)
-          movement.edit(
-            start_date=DateTime(((stop_date-max_delay).Date())),
-            stop_date=DateTime(stop_date.Date()),
-            quantity=max(min_flow, -inventory_item.inventory),
-            quantity_unit=resource.getQuantityUnit()
-            # XXX FIXME define on a supply line
-            # quantity_unit
-          )
-          movement_list.append(movement)
+          if stop_date != None:
+            max_delay = resource.getMaxDelay(0)
+            movement.edit(
+              start_date=DateTime(((stop_date-max_delay).Date())),
+              stop_date=DateTime(stop_date.Date()),
+              quantity=max(min_flow, -inventory_item.inventory),
+              quantity_unit=resource.getQuantityUnit()
+              # XXX FIXME define on a supply line
+              # quantity_unit
+            )
+            movement_list.append(movement)
     return movement_list
 
   def _searchMovementList(self, **kw):