Reduce blast-radius of UNSET in default map
Reduce blast-radius of UNSET in default map: a task in repo2rlenv-pr-runtime (Harbor dataset). Title: lookup default returns Sentinel.UNSET instead of None
The task
**Title:** `lookup_default` returns `Sentinel.UNSET` instead of `None`
Part of FineEnvs/repo2rlenv-pr-runtime.