add global 'empty' property, this property raise mandatory PropertiesOptionError if multi or master have empty value

tiramisu/storage/dictionary/option.py

@@ -79,7 +79,7 @@ class StorageBase(object):
             self._properties = properties
         if opt is not undefined:
             self._opt = opt
-        if allow_empty_list is not False:
+        if allow_empty_list is not undefined:
             self._allow_empty_list = allow_empty_list
 
     def _set_default_values(self, default, default_multi):
@@ -302,8 +302,7 @@ class StorageBase(object):
         try:
             return self._allow_empty_list
         except AttributeError:
-            return False
-
+            return undefined
 
     def _get_extra(self, key):
         extra = self._extra