Function reassemble_with_na 

fn reassemble_with_na<T: Clone>(
    sorted: Vec<Option<T>>,
    na_count: usize,
    na_last: Option<bool>,
) -> Vec<Option<T>>

Reassemble sorted non-NA values with NAs placed according to na_last.

• Some(true) — NAs go at end
• Some(false) — NAs go at beginning
• None — NAs are removed