use if let Some(x) = arr.dim() instead of checking is_array

[daniellga]

fn new_from_values(arr: Robj, dtype: &HDataType) -> Robj {
    assert!(arr.is_array());

    // Ok to unwrap since it was checked that robj is an array.
    let mut dim: Vec<usize> = arr
        .dim()
        .unwrap()
        .iter()
        .map(|z| z.inner() as usize)
        .collect();
    dim.reverse();