Make returning Code nullable

[leoshusar]

Since jsQR returns null if no code has been decoded, this library should return nullable Code.

If you write this:

Code? code = jsQR(imgData.data, canvas.width, canvas.height);
if (code != null) {
  print(code.data);
}

dart2js in release mode optimizes out the null check, because jsQR returns non nullable object and then it tries to print(code.data); even though code is null.