unit_price and quantity could be None during on_change calls

issue2740

purchase.py

@@ -356,8 +356,8 @@ class Purchase(Workflow, ModelSQL, ModelView):
 
                 with Transaction().set_context(context):
                     tax_list = tax_obj.compute(line.get('taxes', []),
-                            line.get('unit_price', Decimal('0.0')),
-                            line.get('quantity', 0.0))
+                            line.get('unit_price') or Decimal('0.0'),
+                            line.get('quantity') or 0.0)
                 for tax in tax_list:
                     key, val = invoice_obj._compute_tax(tax, 'in_invoice')
                     if not key in taxes:
@@ -1068,7 +1068,7 @@ class PurchaseLine(ModelSQL, ModelView):
             context2['purchase_date'] = vals['_parent_purchase.purchase_date']
         with Transaction().set_context(context2):
             res['unit_price'] = product_obj.get_purchase_price([product.id],
-                    vals.get('quantity', 0))[product.id]
+                    vals.get('quantity') or 0)[product.id]
             if res['unit_price']:
                 res['unit_price'] = res['unit_price'].quantize(
                     Decimal(1) / 10 ** self.unit_price.digits[1])
@@ -1139,7 +1139,7 @@ class PurchaseLine(ModelSQL, ModelView):
             context['uom'] = vals['unit']
         with Transaction().set_context(context):
             res['unit_price'] = product_obj.get_purchase_price(
-                    [vals['product']], vals.get('quantity', 0)
+                    [vals['product']], vals.get('quantity') or 0
                     )[vals['product']]
             if res['unit_price']:
                 res['unit_price'] = res['unit_price'].quantize(