Fix type error bug.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@13075 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Form/DurationField.py

@@ -105,7 +105,7 @@ class DurationValidator(Validator.FloatValidator):
                                 validation=1, key=key),
                               REQUEST
                               )
-    if sub_field_value is not None:
+    if sub_field_value not in (None, ''):
       if second_value is not None:
         second_value += sub_field_value * convertion
       else: