How can an absent previewDeploymentsDisabled field be restored after PATCH?

GET /v9/projects/{idOrName} may return a project object in which previewDeploymentsDisabled is absent. The update-project API documents this property as an optional Boolean.

Could Vercel clarify:

What is the effective value when the field is absent?

Is absence semantically and representationally equivalent to false?

After setting it to true, does PATCH with false restore the original absent state?

Is null supported as an unset operation?

Does omitting the property from a later PATCH leave it unchanged or remove it?

Is there another documented operation that restores field absence?

I am seeking documented API semantics, not instructions to test this against a live project.