REF: Fix mypy v0.920 issue

Ref: python/mypy#9430

Author: kernc

backtesting/lib.py

@@ -286,7 +286,7 @@ def func(x, *_, **__):
         series = series.s
 
     if agg is None:
-        agg = OHLCV_AGG.get(getattr(series, 'name', None), 'last')
+        agg = OHLCV_AGG.get(getattr(series, 'name', ''), 'last')
         if isinstance(series, pd.DataFrame):
             agg = {column: OHLCV_AGG.get(column, 'last')
                    for column in series.columns}