ERP5Workflow: fix wrong 'None' value for non defined expression

product/ERP5Workflow/mixin/guardable.py

@@ -70,7 +70,7 @@ class GuardableMixin(object):
           break
       else:
         return False
-    if self.guard_expression is not None:
+    if self.guard_expression.text:
       expression_context = createExprContext(StateChangeInfo(current_object,
                                                              workflow,
                                                              kwargs=kw))