moat: incorrect response if no bridges available

Kathy and I are working on Moat client edge cases today :) If a transport type that BridgeDB supports is requested and the correct CAPTCHA solution is provided in a /check request, the response is a moat-bridges response that includes an empty bridges array, like this:

{
  "data": [
    {
      "qrcode": null,
      "bridges": [],
      "version": "0.1.0",
      "type": "moat-bridges",
      "id": 3
    }
  ]
}

However, the Moat protocol spec says we will receive a 404 error response. Either response will do the job, but we need to make sure client and server are in agreement.