for a multi mandatory, allow [] with allow_empty_list attribut

tiramisu/setting.py

@@ -467,7 +467,7 @@ class Settings(object):
         else:
             if 'mandatory' in properties and \
                     not self._getcontext().cfgimpl_get_values()._isempty(
-                        opt_or_descr, value):
+                        opt_or_descr, value, opt_or_descr.impl_allow_empty_list()):
                 properties.remove('mandatory')
             if is_write and 'everything_frozen' in self_properties:
                 properties.add('frozen')