Ref #1012: investigate unknown data

[leplatrem]

Ref #1012

This test seems to contain the same data as https://mozilla.sentry.io/issues/4087704376/?project=6260602&query=is%3Aunresolved&stream_index=0

but does not produce the same warning :(

ctms.to_vendor() could not convert unknown data

{
  unknown_data: {
    fpn_country: ua
  }
}