unary kernel doesn't respect offset

[bjchambers]

Describe the bug

unary kernel doesn't respect the offset when determining the null buffer, which means that the results are incorrect when applied to a slice.

To Reproduce
Steps to reproduce the behavior:

 #[test]
    fn test_unary_f64_slice() {
        let input =
            Float64Array::from(vec![Some(5.1f64), None, Some(6.8), None, Some(7.2)]);
        let input_slice = input.slice(1, 4);
        let input_slice: &Float64Array = as_primitive_array(&input_slice);
        let result = unary(input_slice, |n| n.round());
        assert_eq!(
            result,
            Float64Array::from(vec![None, Some(7.0), None, Some(7.0)])
        )
    }

Expected behavior

The test should pass.

Actual behavior

The test fails:

thread 'compute::kernels::arity::tests::test_unary_f64_slice' panicked at 'assertion failed: `(left == right)`
  left: `PrimitiveArray<Float64>
[
  0.0,
  null,
  0.0,
  null,
]`,
 right: `PrimitiveArray<Float64>
[
  null,
  7.0,
  null,
  7.0,
]`', arrow/src/compute/kernels/arity.rs:88:9

Additional context