Check None before going through iteration (#13250)

* Check None before going through iteration

* Format

src/transformers/trainer.py

@@ -1411,7 +1411,9 @@ class Trainer:
         load_result = self.model.load_state_dict(state_dict, strict=False)
 
         if len(load_result.missing_keys) != 0:
-            if set(load_result.missing_keys) == set(self.model._keys_to_ignore_on_save):
+            if self.model._keys_to_ignore_on_save is not None and set(load_result.missing_keys) == set(
+                self.model._keys_to_ignore_on_save
+            ):
                 self.model.tie_weights()
             else:
                 logger.warn(f"There were missing keys in the checkpoint model loaded: {load_result.missing_keys}.")