refactor: Make '_cts' nullable type

Author: chainmeJB

NetSdrClientApp/Networking/TcpClientWrapper.cs

@@ -16,7 +16,7 @@ public class TcpClientWrapper : ITcpClient
         private readonly int _port;
         private TcpClient? _tcpClient;
         private NetworkStream? _stream;
-        private CancellationTokenSource _cts;
+        private CancellationTokenSource? _cts;
 
         public bool Connected => _tcpClient != null && _tcpClient.Connected && _stream != null;