change None to undefined when it's needed

tiramisu/option/baseoption.py

@@ -24,7 +24,7 @@ from types import FunctionType
 import warnings
 
 from tiramisu.i18n import _
-from tiramisu.setting import log
+from tiramisu.setting import log, undefined
 from tiramisu.autolib import carry_out_calculation
 from tiramisu.error import ConfigError, ValueWarning
 
@@ -164,14 +164,14 @@ class BaseOption(object):
         """
         self._impl_informations[key] = value
 
-    def impl_get_information(self, key, default=None):
+    def impl_get_information(self, key, default=undefined):
         """retrieves one information's item
 
         :param key: the item string (ex: "help")
         """
         if key in self._impl_informations:
             return self._impl_informations[key]
-        elif default is not None:
+        elif default is not undefined:
             return default
         else:
             raise ValueError(_("information's item not found: {0}").format(