ATLASRECTS-7348: Return FAILURE if primary vertices are empty

We don't expect an empty primary vertex container in events we b-tag, but for some reason we were returning StatusCode::SUCCESS when we had one. This will do unpredictable things in downstream code.

This change means we'll throw an error early on if the vertex collection is empty.

It's extremely unlikely that this will cause problems: there is already an algorithm that returns FAILURE on events without primary vertices which runs as part of standard b-tagging.