If a face is not detected in a frame, bbox will be empty. So you can simply check
if isempty(bbox) ...
to handle the case when a face is not detected.
Generally, you probably want to track the face, because vision.CascadeObject detector is not likely to detect a face, which is tilted or turned. Please see this example of tracking a face using the KLT algorithm, and this example of tracking multiple faces .
