[skeleton] correctly catch zero len detection

fixtracks/widgets/skeleton.py

@@ -165,6 +165,8 @@ class SkeletonWidget(QWidget):
                      frames:np.ndarray, tracks:np.ndarray, brush:QBrush):
         num_detections = 0 if coordinates is None else coordinates.shape[0]
         logging.debug("SkeletonWidget: add %i Skeletons", num_detections)
+        if num_detections < 1:
+            return
         sorting = np.argsort(frames)
         coordinates = coordinates[sorting,:, :]
         detection_ids = detection_ids[sorting]