ERP5Workflow: if status value is None, don't change it to empty string!

in the Workflow, the status dict (used to set history) can handle None
values, and the behaviour should not differ from DCWorkflow.

This should fix SyncML test failures.

product/ERP5SyncML/Conduit/ERP5Conduit.py

@@ -902,7 +902,7 @@ class ERP5Conduit(XMLSyncUtilsMixin):
     addable = WORKFLOW_ACTION_ADDABLE
     time = status.get('time')
     for action in action_list:
-      this_one = WORKFLOW_ACTION_ADDABLE
+      this_one = True
       if time <= action.get('time'):
         # action in the past are not appended
         addable = WORKFLOW_ACTION_INSERTABLE
@@ -910,7 +910,7 @@ class ERP5Conduit(XMLSyncUtilsMixin):
       key_list.remove("time")
       for key in key_list:
         if status[key] != action[key]:
-          this_one = WORKFLOW_ACTION_NOT_ADDABLE
+          this_one = False
           break
       if this_one:
         addable = WORKFLOW_ACTION_NOT_ADDABLE

product/ERP5Workflow/Document/Workflow.py

@@ -717,7 +717,6 @@ class Workflow(IdAsReferenceMixin("", "prefix"), XMLObject):
           econtext = createExprContext(sci)
         expr = Expression(expr)
         value = expr(econtext)
-      if value is None: value = ''
       status_dict[variable_reference] = value
     # Do not proceed in case of failure of before script
     if not before_script_success: