Make `check-migrations` exit gracefully for kit workspaces

If you run `twoliter make check` in a workspace that only includes kits, [`check-migrations`](https://github.com/bottlerocket-os/twoliter/blob/82c19cd9c60a288aaf194e6c68bb934c3938e7b8/twoliter/embedded/Makefile.toml#L398) will run and fail due to a lack of migrations present in the repository.

This is an artifact of splitting kits from the `bottlerocket-os/bottlerocket` repository. We should let this task exit gracefully in cases where migrations are not needed anymore.

Start in twoliter/embedded/Makefile.toml at the linked check-migrations task, then run twoliter make check in a workspace containing only kits to reproduce the failure. Adjust the task so migrations are treated as unnecessary when none are present, and verify that the kit-only check completes successfully without changing migration behavior for repositories that have migrations.

Written by the indexing model from the issue text.