Function coalesce_nulls

pub fn coalesce_nulls<'a, T: PolarsDataType>(
    a: &'a ChunkedArray<T>,
    b: &'a ChunkedArray<T>,
) -> (Cow<'a, ChunkedArray<T>>, Cow<'a, ChunkedArray<T>>)

ensure that nulls are propagated to both arrays