Return a string if `self.as_string=True`, otherwise return this field's `num_type`.

def _serialize(self, value, attr, obj, **kwargs):
        """Return a string if `self.as_string=True`, otherwise return this field's `num_type`."""
        ret = self._validated(value)
        return (
            self._to_string(ret)
            if (self.as_string and ret not in (None, missing_))
            else ret
        )