Fix mock in test_cached_files_are_used_when_internet_is_down (#18804)

tests/test_modeling_common.py

@@ -2931,6 +2931,7 @@ class ModelUtilsTest(TestCasePlus):
         response_mock.status_code = 500
         response_mock.headers = {}
         response_mock.raise_for_status.side_effect = HTTPError
+        response_mock.json.return_value = {}
 
         # Download this model to make sure it's in the cache.
         _ = BertModel.from_pretrained("hf-internal-testing/tiny-random-bert")