[run_clm.py] fix getting extention

Author: patil-suraj

examples/language-modeling/run_clm.py

@@ -227,7 +227,11 @@ def main():
             data_files["train"] = data_args.train_file
         if data_args.validation_file is not None:
             data_files["validation"] = data_args.validation_file
-        extension = data_args.train_file.split(".")[-1]
+        extension = (
+            data_args.train_file.split(".")[-1]
+            if data_args.train_file is not None
+            else data_args.validation_file.split(".")[-1]
+        )
         if extension == "txt":
             extension = "text"
         datasets = load_dataset(extension, data_files=data_files)