Function pad_left 

pub fn pad_left<R, O, N>(
    right: &R,
    left_width: usize,
    row_id: u64,
    null_value: N,
    build_row: impl FnOnce(u64, Vec<R::Value>) -> O,
) -> O
where
    R: RowLike,
    N: FnMut() -> R::Value,

Combine a right row with left-side null padding.