Fix load_results for None data frames

src/results.py

@@ -50,6 +50,7 @@ async def load_results_dataframe(model_id, result_paths_per_model=None):
 
 async def load_results(model_ids, result_paths_per_model=None):
     dfs = await asyncio.gather(*[load_results_dataframe(model_id, result_paths_per_model) for model_id in model_ids])
+    dfs = [df for df in dfs if df is not None]
     if dfs:
         return pd.concat(dfs)