[senknvd]

I don't think RFC 4180 differentiates between an empty string and a null value. As long as you add a check that all string columns are free of empty values before writing you should be good.

I think in polars it's

    df.filter(pl.col(pl.Utf8).str.len_bytes() == 0).shape[0] == 0

although there's probably a better way to write this.

[nradov]

Well I would consider differentiation between empty string versus null as simply being out of scope for CSV rather than undefined behavior. It was never intended as a complete database dump format.