code: def field2nullable(self, field, **kwargs):
          
          attributes = {}
          if field.allow_none:
              attributes[
                  "x-nullable" if self.openapi_version.major < 3 else "nullable"
              ] = True
          return attributes
docs: Return the dictionary of OpenAPI field attributes for a nullable field.
  
          :param Field field: A marshmallow field.
          :rtype: dict