Fix issue with legacy mode responding with an error when it should pass

[JackLewis-digirati]

This request:

{
  "origin": "https://dlcsstage-public-test-objects.s3.eu-west-1.amazonaws.com/other-video/fish.mp4",
  "mediaType": "video/mp4",
  "deliveryChannels": [
    {
      "channel": "iiif-av",
      "policy": "default-video"
    }
  ]
}

causes the below response in legacy mode:

{
  "@type": "Error",
  "statusCode": 400,
  "status": 400,
  "title": "Bad request",
  "detail": "'deliveryChannels' cannot be an empty array",
  "instance": "https://api.dlcs-stage.digirati.io/",
  "description": "'deliveryChannels' cannot be an empty array"
}

This is due to previously treating null and empty the same way when working out the delivery channels based on media type. However, this has changed with #944 where empty is treated as a 400. As such, this PR modifies the response of the LegacyConverter to correctly respond with null, rather than empty when the delivery channels cannot be worked out directly