#854: Reject empty just_one matching criteria

[gemshrine]

Fbe.just_one builds its lookup query from the attributes set by the callback. When the callback leaves no ordinary attributes, or sets only system attributes, the query becomes (and ); that can return an unrelated fact or fail in the parser.

The method now checks the non-system matching attributes after the callback and raises Fbe::Error before querying or inserting when the set is empty. Existing attribute handling and matching behavior stay the same when a usable key is present.

Tests cover an empty callback with an existing unrelated fact and a callback that sets only _id. The repository test suite runs these cases with bundle exec rake test.

Fixes #854