Add val.py no label warning (#8782)

Help resolve confusion around zero-metrics val.py results when no labels are found in https://github.com/ultralytics/yolov5/issues/8753

val.py

@@ -275,6 +275,8 @@ def run(
     # Print results
     pf = '%20s' + '%11i' * 2 + '%11.3g' * 4  # print format
     LOGGER.info(pf % ('all', seen, nt.sum(), mp, mr, map50, map))
+    if nt.sum() == 0:
+        LOGGER.warning(emojis(f'WARNING: no labels found in {task} set, can not compute metrics without labels ⚠️'))
 
     # Print results per class
     if (verbose or (nc < 50 and not training)) and nc > 1 and len(stats):