Fix "edit row" logic for ordering batch

previous logic allowed `colander.null` to be passed to batch handler, which
caused an error.  also it allowed editing "all" fields for the row, which we
really don't need to do, so now we just support the order quantities

tailbone/views/purchasing/batch.py

@@ -922,9 +922,19 @@ class PurchasingBatchView(BatchMasterView):
 
         if batch.mode == self.enum.PURCHASE_BATCH_MODE_ORDERING:
 
-            # let handler update data, per given order quantities
-            data = self.form_deserialized
-            self.handler.update_row_quantity(row, **data)
+            # figure out which values need updating
+            form_data = self.form_deserialized
+            data = {}
+            for key in ('cases_ordered', 'units_ordered'):
+                if key in form_data:
+                    # this is really to convert/avoid colander.null, but the
+                    # handler method also assumes that if we pass a value, it
+                    # will not be None
+                    data[key] = form_data[key] or 0
+            if data:
+
+                # let handler do the actual updating
+                self.handler.update_row_quantity(row, **data)
 
         else: # *not* ordering mode