ERP5Workflow: override accessor private methods

the private setters are called by methods like 'edit', ...

product/ERP5Workflow/mixin/guardable.py

@@ -140,12 +140,13 @@ class GuardableMixin(object):
 
     return changed
 
-  security.declareProtected(ManagePortal, 'setGuardExpression')
-  def setGuardExpression(self, text):
+  def _setGuardExpression(self, text):
+    if text:
       self.guard_expression = Expression(text)
+    else:
+      self.guard_expression = None
 
-  security.declareProtected(ManagePortal, 'getGuardExpression')
-  def getGuardExpression(self):
+  def _getGuardExpression(self):
     if self.guard_expression is None:
       return Expression('')
     return self.guard_expression