Added support for optional columns in db

src/admin/common/abstract/model/factory.php

@@ -30,6 +30,7 @@ abstract class AbstractCommonClubsModelFactory
      *      - int
      *      - float
      *      - ref
+     *  - optional: boolean, if true, the field can be NULL
      *  - ref: (only with type='ref') The name of the class that is referenced
      */
     public abstract function getAttributes();