Fix confusion matrix update when no predictions are made (#8748)

* Fix confusion matrix update when no predictions are made

* Update metrics.py

* Simply confusion matrix changes

* Simply confusion matrix fix

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

val.py

@@ -228,6 +228,8 @@ def run(
             if npr == 0:
                 if nl:
                     stats.append((correct, *torch.zeros((2, 0), device=device), labels[:, 0]))
+                    if plots:
+                        confusion_matrix.process_batch(detections=None, labels=labels[:, 0])
                 continue
 
             # Predictions