Return the dictionary of OpenAPI field attributes for a nullable field.

        :param Field field: A marshmallow field.
        :rtype: dict

def field2nullable(self, field, **kwargs):
        """Return the dictionary of OpenAPI field attributes for a nullable field.

        :param Field field: A marshmallow field.
        :rtype: dict
        """
        attributes = {}
        if field.allow_none:
            attributes[
                "x-nullable" if self.openapi_version.major < 3 else "nullable"
            ] = True
        return attributes