minor TypeError fix

[maximilianmozes]

During validation, ObjectDetector.nms_boxes returns None if “nothing was detected“ (line 402 in lib/object_detector.py). This yields a TypeError in ObjectDetector.forward when unrolling the returned objects (line 308 in lib/object_detector.py).

[GreenTeaHua]

thanks,
but after your fix, in res_model.py, meet this error.
if result.is_none():
return ValueError("heck")