extract value

        extarct value from querydict and convert it to native
        missing and empty values result to None

def parse_value(self, querydict):
        """ extract value

        extarct value from querydict and convert it to native
        missing and empty values result to None
        """
        value = self.field.get_value(querydict)
        if value in (None, fields.empty, ''):
            return None
        return self.field.to_internal_value(value)