make facedetect.py work when the nested cascade is not specified

samples/python2/facedetect.py

@@ -49,6 +49,7 @@ if __name__ == '__main__':
         rects = detect(gray, cascade)
         vis = img.copy()
         draw_rects(vis, rects, (0, 255, 0))
+        if not nested.empty():
             for x1, y1, x2, y2 in rects:
                 roi = gray[y1:y2, x1:x2]
                 vis_roi = vis[y1:y2, x1:x2]