discard also empty strings, happens when Contribution Predicate is just empty

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@27398 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Tool/ContributionRegistryTool.py

+# -*- coding: utf-8 -*-
 #############################################################################
 #
 # Copyright (c) 2008 Nexedi SA and Contributors. All Rights Reserved.
@@ -48,7 +49,7 @@ class ContributionRegistryTool(BaseTool):
 
     for predicate in self.objectValues(sort_on='int_index'):
       result = predicate.test(ingestion_file)
-      if not result in (None, False):
+      if result:
         return result
 
 InitializeClass(ContributionRegistryTool)