check that quantity is not None before return -quantity

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@18167 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Document/LeavePeriod.py

@@ -69,4 +69,7 @@ class LeavePeriod(PresencePeriod):
     quantity = PresencePeriod.getInventoriatedQuantity(
                                             self, default=default,
                                             *args, **kw)
-    return -quantity
+    if quantity is not None:
+      return -quantity
+    else:
+      return None