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If you check the code, image_converter.cpp the images are shown in line 53

  51     // Update GUI Window
  52     cv::imshow(OPENCV_WINDOW, cv_ptr->image);
  53     cv::waitKey(3);

cv::waitKey handles any windowing events, such as creating windows or showing images. if you call cv::imshow() in a loop through video frames, without following up each draw with cv::waitKey(3) nothing appears on screen, because highgui is never given time to process the draw requests from cv::imshow(). If you insist on showing an empty window before getting the images add

cv::waitKey(3);

after

cv::namedWindow("test",1);

you can replace 3 by any other small number, it expresses how long opencv should wait in milliseconds.