Add support for zero quantity for mobile inventory batch rows

tailbone/views/inventory.py

@@ -588,12 +588,14 @@ class InventoryBatchView(BatchMasterView):
                 row = self.Session.query(model.InventoryBatchRow).get(update_form.validated['row'])
                 cases = update_form.validated['cases']
                 units = update_form.validated['units']
-                if cases:
+                if cases is not colander.null:
                     row.cases = cases
                     row.units = None
-                elif units:
+                elif units is not colander.null:
                     row.cases = None
                     row.units = units
+                else:
+                    raise NotImplementedError
                 self.handler.refresh_row(row)
                 route_prefix = self.get_route_prefix()
                 return self.redirect(self.request.route_url('mobile.{}.view'.format(route_prefix), uuid=batch.uuid))