erp5_credential: Don"t set start date on assignments if stop date is None

bt5/erp5_credential/SkinTemplateItem/portal_skins/erp5_credential/CredentialRequest_updatePersonAssignment.py

@@ -23,8 +23,11 @@ assignment_duration = context.portal_preferences.getPreferredCredentialAssignmen
 today = DateTime()
 if assignment_duration < 0:
   delay = None
+  start_date = None
 else:
   delay = today+assignment_duration
+  start_date = today - 1
+
 
 current_assignment_list = {}
 for assignment in open_assignment_list:
@@ -57,7 +60,7 @@ for role in context.getRoleList():
         function = context.getFunctionList(),
         group = context.getGroupList(),
         destination = organisation_url,
-        start_date = today - 1,
+        start_date = start_date,
         stop_date = delay)
 
     assignment.open()