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

tiramisu/option/baseoption.py

@@ -99,7 +99,7 @@ class Base(StorageBase):
     def __init__(self, name, doc, default=None, default_multi=None,
                  requires=None, multi=False, callback=None,
                  callback_params=None, validator=None, validator_params=None,
-                 properties=None, warnings_only=False, extra=None, allow_empty_list=False):
+                 properties=None, warnings_only=False, extra=None, allow_empty_list=undefined):
         if not valid_name(name):  # pragma: optional cover
             raise ValueError(_("invalid name: {0} for option").format(name))
         if requires is not None:
@@ -900,7 +900,7 @@ class SymLinkOption(OnlyOption):
                                'for symlink {0}').format(name))
         super(Base, self).__init__(name, undefined, undefined, undefined,
                                    undefined, undefined, undefined, undefined,
-                                   False, opt)
+                                   undefined, opt)
         self.commit()
 
     def __getattr__(self, name, context=undefined):