InteractionWorkflow.py: in showAsXML deploy expression getter; turn...

InteractionWorkflow.py: in showAsXML deploy expression getter; turn property_value into empty string when it is an empty tuple.

product/ERP5Workflow/Document/InteractionWorkflow.py

@@ -488,7 +488,7 @@ class InteractionWorkflow(IdAsReferenceMixin("", "prefix"), XMLObject):
             property_value = tuple(property_value)
           sub_object = SubElement(guard, property_id, attrib=dict(type='guard configuration'))
         elif property_id == 'expr':
-          property_value = tdef.getProperty(property_id)
+          property_value = tdef.getExpression()
           sub_object = SubElement(guard, property_id, attrib=dict(type='guard configuration'))
         # no-property definded action box configuration
         elif property_id in ['actbox_name', 'actbox_url', 'actbox_category', 'trigger_type']:
@@ -519,7 +519,7 @@ class InteractionWorkflow(IdAsReferenceMixin("", "prefix"), XMLObject):
           property_value = tdef.getProperty(property_id)
           property_type = tdef.getPropertyType(property_id)
           sub_object = SubElement(interaction, property_id, attrib=dict(type=property_type))
-        if property_value is None or property_value == []:
+        if property_value is None or property_value == [] or property_value == ():
           property_value = ''
         sub_object.text = str(property_value)