internal error while describing legacy attestation #184

#180 introduced a bug in the attestation fetching code for attestations stored previously to the change.

The issue is that the server runs into a nil pointer exception instead of returning an empty attestation object (like before)

 chainloop wf run describe --id a9f2a5e7-a99f-4041-a9ee-a7a8f45df126
ERR there was an internal error

The error is raised here

/home/runner/work/chainloop/chainloop/app/controlplane/internal/service/attestation.go in bizAttestationToPb at line 247