new_null_array generates an invalid union array

[kawadakk]

Describe the bug
new_null_array generates a union array that fails validation by ArrayData::validate_full.

To Reproduce
Modify test_null_union as follows:

#[test]
fn test_null_union() {
    for mode in [UnionMode::Sparse, UnionMode::Dense] {
        let data_type = DataType::Union(
            UnionFields::new(
                vec![2, 1],
                vec![
                    Field::new("foo", DataType::Int32, true),
                    Field::new("bar", DataType::Int64, true),
                ],
            ),
            mode,
        );
        let array = new_null_array(&data_type, 4);

        let array = as_union_array(array.as_ref());
        assert_eq!(array.len(), 4);
        assert_eq!(array.null_count(), 0);

        for i in 0..4 {
            let a = array.value(i);
            assert_eq!(a.len(), 1);
            assert_eq!(a.null_count(), 1);
            assert!(a.is_null(0))
        }

        array.to_data().validate_full().unwrap(); // <---- new
    }
}

This will fail with the following output: thread 'array::tests::test_null_union' panicked at 'called `Result::unwrap()` on an `Err` value: InvalidArgumentError("Sparse union child array #1 has length smaller than expected for union array (0 < 4)")', arrow-array\src\array\mod.rs:852:45

Expected behavior
Producing a valid array

Additional context

[tustvold]

label_issue.py automatically added labels {'arrow'} from #4601