return Json with value only two brackets id there no data

Check the below example, this may help you.

public IHttpActionResult GetApiResponse(bool condition)
{
    try
    {
        var evidence = new EvidenceObj
        {
            File = "JVBERi0xLjQNCjEgMCBvYmoNCjw8DQovQ3JlYXRvciAoT3JhY2xlMTFnUjEgQVMgUmVwb3J0cyBTZXJ2aWNlcykNCi9DcmVhdG",
            FileType = "pdf"
        };
        ApiResponse response = null;
        if (condition)
            response = new ApiResponse { Evidence = evidence };
        else
            response = new ApiResponse { Evidence = new EvidenceObj() };

        return new RawJsonActionResult(JsonConvert.SerializeObject(response));
    }
    catch (Exception e)
    {
        logger.Error(e.Message);
        throw new HttpResponseException(new HttpResponseMessage(HttpStatusCode.InternalServerError)
        {
            Content = new StringContent("Error : " + e.Message),
            ReasonPhrase = "Critical Exception"
        });
    }
}

Model class:

public class EvidenceObj
{
    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public string File { get; set; }
    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public string FileType { get; set; }
}

public class ApiResponse
{
    public EvidenceObj Evidence { get; set; }
}