Fix `is_coco` on missing `data['val']` key (#4642)

val.py

@@ -134,7 +134,7 @@ def run(data,
 
     # Configure
     model.eval()
-    is_coco = type(data['val']) is str and data['val'].endswith('coco/val2017.txt')  # COCO dataset
+    is_coco = isinstance(data.get('val'), str) and data['val'].endswith('coco/val2017.txt')  # COCO dataset
     nc = 1 if single_cls else int(data['nc'])  # number of classes
     iouv = torch.linspace(0.5, 0.95, 10).to(device)  # iou vector for mAP@0.5:0.95
     niou = iouv.numel()