switch (response.StatusCode) {
  //200
  case (System.Net.HttpStatusCode.OK):

    HttpContent content = response.Content;
    string xjson = await content.ReadAsStringAsync();

    try {
      //List<Table_Loc> loc_list = JsonConvert.DeserializeObject<List<Table_Loc>>(xjson);
      lbl_res.Text = "Ordenes Pendientes";
      Root myobject = JsonConvert.DeserializeObject < Root > (xjson);

      //var T1count = myobject.tablas.Table1.Count;

      if (myobject.tablas.Table1.Count == 0) {
        PasajeAereo.IsEnabled = false;
      } else {
        PasajeAereo.IsEnabled = true;
      }


    } catch (Exception ex) {
      await DisplayAlert("", "" + ex.ToString(), "ok");
      return;
    }

    break;
}

//Model ...

public class DatosEnvio
{
}

public class DatosEnvioJson
{
}

public class Table1
{
    public string aerolinea { get; set; }
    public string vuelo { get; set; }
    public string salida { get; set; }
    public DateTime sFecha { get; set; }
    public string sHora { get; set; }
    public string regreso { get; set; }
    public DateTime rFecha { get; set; }
    public string rHora { get; set; }
    public string motivo { get; set; }
    public int idOrden { get; set; }
}

public class Tablas
{
    public List<Table1> Table1 { get; set; }
}

public class Root
{
    public DatosEnvio DatosEnvio { get; set; }
    public DatosEnvioJson DatosEnvioJson { get; set; }
    public Tablas tablas { get; set; }
    public string bandera { get; set; }
    public string mensaje { get; set; }
}

if (myobject != null && myobject.tablas != null && myobject.tablas.Table1 != null &&  myobject.tablas.Table1.Count == 0) {
        PasajeAereo.IsEnabled = false;
      } else {
        PasajeAereo.IsEnabled = true;
      }

if (myobject?.tablas?.Table1?.Count == (int?)0) {
        PasajeAereo.IsEnabled = false;
 }
  else {
        PasajeAereo.IsEnabled = true;
 }

PasajeAereo.IsEnabled = myobject?.tablas?.Table1?.Count == (int?)0;