Follow-up from "Resolve instance-level SSH CAs in /internal/authorized_certs"

The following discussion from !253556 (merged) should be addressed:

• @vyaklushin started a discussion:

  nitpick We can remove nil-handling and simplify the code if we return an error message here.

  def group_ssh_certificate_response
    ServiceResponse.error(message: 'Certificate Not Found', reason: ::Gitlab::SshCertificates::Reason::CERTIFICATE_NOT_FOUND)
  end