Fix schema validation mis-alignment with rails

Why is this change being made?

Our existing tests perform schema validation differently from how Rails does, due to using two different libraries with differing implementations gitlab-org/gitlab#348840 (comment 784541157)

This MR updates Gcs to use the same schema validation gem as Rails, and also fixes cases where .vulnerabilities[].identifiers[].url may be blank, which causes Rails schema validation to fail.