background_tasks: restore_tenant stores "" for the task owner, which leaks between signal tests #861

Found while moving `background_tasks` to the module layout (#859). Under the new collection order, a signals test ran its prerun handler without a postrun. That left `_task_owner` at `""`, and owner stamping then failed in four later tests. #859 makes that test call `release_tenant(task_id=...)`, the same call the real postrun handler makes.

The underlying fragility is in the source: `restore_tenant` sets the owner to `""` where `None` would fall through to `current_user_id`. A worker that runs a prerun without its matching postrun (crash, revoked task) could therefore stamp the wrong owner on the next task in that thread. Worth checking whether `None` is the intended reset value and adding a regression test.

https://claude.ai/code/session_01FXHA3ZicrtZ4C5QPEgxfgq