Adjust SBoM ingestion pipeline to use `none` type when PURL is empty

Why are we doing this work?

After introducing a new type in Add `none` to PURL types (#495101 - closed) we should adjust our ingestion pipeline so that we no longer insert rows where purl_type IS NULL

Implementation plan

1. Adjust #purl_type in ee/app/services/sbom/ingestion/occurrence_map.rb to return "none" if empty