<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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