Incorrect null count for cast kernel for list arrays

[alamb]

Describe the bug
When casting a ListArray using the cast kernel, the resulting null count is incorrect

To Reproduce

use std::sync::Arc;

use arrow::{array::{Array, ArrayRef, ListArray}, datatypes::{DataType, Field, Int32Type, UInt32Type}};

fn main() {

    // Form the list [[0, -11, -22], null]
    let data = vec![
        Some(vec![Some(0), Some(-11), Some(-22)]),
        None,
    ];
    let list_array = ListArray::from_iter_primitive::<Int32Type, _, _>(data);

    assert_eq!(DataType::Int32, list_array.value_type());
    assert_eq!(2, list_array.len());
    assert_eq!(1, list_array.null_count());

    let list_array: ArrayRef = Arc::new(list_array);

    // Cast to UInt32, resulting in
    // the list [[0, null, null], null]
    let new_datatype = DataType::List(Box::new(Field::new("item", DataType::UInt32, true)));
    let cast_array = arrow::compute::cast(&list_array, &new_datatype).unwrap();
    let cast_array = cast_array.as_any().downcast_ref::<ListArray>().unwrap();

    let data = vec![
        Some(vec![Some(0), None, None]),
        None,
    ];
    let expected_array = ListArray::from_iter_primitive::<UInt32Type, _, _>(data);

    assert_eq!(expected_array.null_count(), 1);
    // currently fails cast_array has a null count of 2 (not 1)
    assert_eq!(cast_array.null_count(), 1);
    // the expected array should be the same as well
    assert_eq!(cast_array, &expected_array);
}

Results in this failure:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `2`,
 right: `1`', src/main.rs:34:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Expected behavior
The test program above should pass

Additional context
Found while adding validation to #810