Handle empty dataframes (which occur more commonly now that get_price…

…_data_from_polygon will return them).

lumibot/backtesting/polygon_backtesting.py

@@ -174,15 +174,12 @@ def update_pandas_data(self, asset, quote, length, timestep, start_dt=None, upda
             logging.error(traceback.format_exc())
             raise Exception("Error getting data from Polygon") from e
 
-        if df is None:
+        if (df is None) or df.empty:
             return
 
         data = Data(asset_separated, df, timestep=ts_unit, quote=quote_asset)
         pandas_data_update = self._set_pandas_data_keys([data])
 
-        if pandas_data_update is None:
-            return
-
         # Add the keys to the self.pandas_data dictionary
         self.pandas_data.update(pandas_data_update)
         # Don't let memory usage get out of control