<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_nullable_string(value):
 """ Converts value into string or returns None when value is None. :param value: the value to convert. :return: string value or None when value is None. """

        if value == None:
            return None
        if type(value) == datetime.date:
            return value.isoformat()
        if type(value) == datetime.datetime:
            if value.tzinfo == None:
                return value.isoformat() + "Z"
            else:
                return value.isoformat()

        if type(value) == list:
            builder = ''
            for element in value:
                if len(builder) > 0:
                    builder = builder + ","
                builder = builder + element
            return builder.__str__()
        return str(value)