NMS --classes 0 bug fix (#1710)

utils/general.py

@@ -311,7 +311,7 @@ def non_max_suppression(prediction, conf_thres=0.25, iou_thres=0.45, classes=Non
             x = torch.cat((box, conf, j.float()), 1)[conf.view(-1) > conf_thres]
 
         # Filter by class
-        if classes:
+        if classes is not None:
             x = x[(x[:, 5:6] == torch.tensor(classes, device=x.device)).any(1)]
 
         # Apply finite constraint