Accept build number zero and empty migration lists in repository checks #32

During a correctness pass with Astra we found the following issue:

Author's preface above. Issue details below written by Astra (AI assistant) on 2026-10-09.

Two valid values are rejected by rs check because it tests truthiness rather than presence/type:

• new-distro writes build_number: 0, but check() reports that the distro has no build number.
• A distro can pin a conda-forge version with conda_forge_migrations: [], but the check requires the migration list to be nonempty.

I verified that Vinca preserves a zero build number unchanged. The rest follows directly from the validation conditions; I did not create a real new distro during the audit. The current four distros pass rs check.

Code: tools/maintenance.py, tools/maintenance.py and tools/maintenance.py.

Suggested change

Validate a nonnegative integer build number and the presence/types of the pinning fields. An empty list is a valid migration list when no migrations apply.

Acceptance criteria

• A new distro with build number zero passes this validation.
• An explicit pinning version with an empty migration list passes.
• Missing build numbers, negative/invalid values and genuinely inconsistent pinning fields still fail with useful messages.
• Regression tests cover these boundaries.