Use empty string for "missing" department name, for ordering worksheet

that way the sorting works as expected (i.e. for python3)

tailbone/views/purchasing/ordering.py

@@ -138,7 +138,7 @@ class OrderingBatchView(PurchasingBatchView):
                 departments.setdefault(department.uuid, department)
             else:
                 if None not in departments:
-                    department = Object(name=None, number=None)
+                    department = Object(name='', number=None)
                     departments[None] = department
                 department = departments[None]