When `inclusion: available` properties are not selected, target fails because properties are not nullable according to the schema

As seen in https://gitlab.com/meltano/meltano/-/jobs/1019512436

commits.short_id is not selected, but since the field is not nullable per https://gitlab.com/meltano/tap-gitlab/-/blob/master/tap_gitlab/schemas/commits.json#L11, target-postgres fails. Other streams likely have the same problem.

We should either exclude unselected properties from the SCHEMA message, or update the schema to allow non-automatic fields to be null.

This ~bug was introduced by !34 (merged), released as v0.9.12.