Single-agency feeds without `agency_id` are rejected

[mattkinnia]

In the GTFS spec, the agency_id field in agency.txt is conditionally required:

• Required when the dataset contains data for multiple transit agencies.
• Recommended otherwise.

Current behaviour:
Single-agency feeds without agency_id are rejected, because agency_id is enforced as a primary key in lib/agency.js.

https://github.com/public-transport/gtfs-via-postgres/blob/cd8fe457fe1583b97aaee5e9fcfae1771ca4c938/lib/agency.js#L6C2-L6C29

Expected behaviour:
Single-agency feeds without agency_id should be accepted as per the GTFS spec.