return quantity even if 0

git-svn-id: https://svn.erp5.org/repos/public/erp5/sandbox/amount_generator@36902 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Document/TradeModelLine.py

@@ -83,8 +83,8 @@ class TradeModelLine(MappedValue, XMLMatrix, Amount, AmountGeneratorMixin):
     result = self._baseGetMappedValuePropertyList()
     if result:
       return result
-    if self._baseGetQuantity(): # If quantity is defined, then tax works as transformed resource
-      return ('quantity', 'price', )
+    if self._baseGetQuantity() is not None: # If quantity is defined, then tax works as transformed resource
+      return ('quantity', 'price', 'step')
     # Else tax provides only a ratio on amount
     return ('price', 'efficiency')