Fix: ultralytics#2674 (ultralytics#2683)

* Set resume flag to false

* Check existance of val dataset

(cherry picked from commit 514ebcd)

utils/wandb_logging/wandb_utils.py

@@ -159,7 +159,7 @@ def setup_training(self, opt, data_dict):
         return data_dict
 
     def download_dataset_artifact(self, path, alias):
-        if path.startswith(WANDB_ARTIFACT_PREFIX):
+        if path and path.startswith(WANDB_ARTIFACT_PREFIX):
             dataset_artifact = wandb.use_artifact(remove_prefix(path, WANDB_ARTIFACT_PREFIX) + ":" + alias)
             assert dataset_artifact is not None, "'Error: W&B dataset artifact doesn\'t exist'"
             datadir = dataset_artifact.download()