Workflow.py: do not accept a None guard.

product/ERP5Workflow/Document/Workflow.py

@@ -204,7 +204,7 @@ class Workflow(IdAsReferenceMixin("", "prefix"), XMLObject):
   def _checkTransitionGuard(self, tdef, document, **kw):
     guard = tdef.getGuard()
     if guard is None:
-      return 1
+      return 0
     if guard.check(getSecurityManager(), self, document, **kw):
       return 1
     return 0