Should we be checking if reservation_id is not None instead of whether it's simply falsey?

Yes, I made this update to follow the same pattern used for the maximum_bytes_billed property.

How come we're setting a field reservation with a property called reservation_id? Are you sure this is the right attribute in the QueryJobConfig instance? Seems like it [contains a property/getter](https://github.com/googleapis/python-bigquery/blob/46764a59ca7a21ed14ad2c91eb7f98c302736c22/google/cloud/bigquery/job/base.py#L540-L550) for accessing the reservation_id.

Agreed, the better name is reservation instead of reservation_id. I updated this also.

Is this the recommended way for setting the reservation ID in the config? What about the QueryJobConfig's constructor?

I updated the _job_params dictionary, it gets passed to QueryJobConfig(**self._job_params) just like priority, and maximum_bytes_billed