polars���sumは全てnullだと0になってしまうのか
https://t.co/yqZ3Ty7LS1
> If there are no non-null values, then the output is 0. If you would prefer empty sums to return None, you can use pl.when(pl.col(name).count()>0).then(pl.sum(name)) instead of pl.sum(name).