How to define model property as nullable in openapi spec

[tokidoki11]

Currently i write

@property({
    type: UserAdditional,
    jsonSchema: {
      nullable: true,
    },
  })
  additional: UserAdditional | null;

it generates {"$ref":"#/components/schemas/UserAdditional"}, with no nullable present
if I remove the type, nullable is present but type is object