Refactor init_leaderboard function to raise ValueError for None DataFrame; improve error handling

app.py

@@ -57,8 +57,8 @@ LEADERBOARD_DF = get_leaderboard_df(EVAL_RESULTS_PATH, EVAL_REQUESTS_PATH, COLS,
 ) = get_evaluation_queue_df(EVAL_REQUESTS_PATH, EVAL_COLS)
 
 def init_leaderboard(dataframe):
-    # if dataframe is None or dataframe.empty:
-    #     raise ValueError("Leaderboard DataFrame is empty or None.")
+    if dataframe is None:
+        raise ValueError("Leaderboard DataFrame is empty or None.")
     return Leaderboard(
         value=dataframe,
         datatype=[c.type for c in fields(AutoEvalColumn())],