Properties do not return anything

Fixes the docs-build job since the newest version of griffe is more strict about class properties.

The properties should not have a :returns: section. I fixed the two that had some and triggered the following warnings in CI

• https://gitlab.com/teklia/workers/base-worker/-/jobs/3466177846#L370
• https://gitlab.com/teklia/workers/base-worker/-/jobs/3466177846#L636