We didn't have any PURL ecosystem validation until [PR
2900](https://github.com/google/osv.dev/pull/2900) was merged. [PR
2900](https://github.com/google/osv.dev/pull/2900) introduced some
[breaking
changes](https://github.com/google/osv.dev/issues/2960#issuecomment-2518943142)
that affect users querying ecosystems for which we currently lack a PURL
converter, especially those not specified in the PURL specification.

This PR changes the response from a 400 error code to a 200 status code
with an empty response unblock users. A more considered design will be
discussed later with the team and implemented.