Boolean not kernel does not take offset of null buffer into account

[jhorstmann]

Describe the bug
The boolean not kernel does use null_buffer.bits.slice(offset) which takes a byte offset. This leads to wrong results and possible panics if the offset is not equal to 0.

To Reproduce

let a = BooleanArray::from(vec![None, Some(true), Some(false), None, Some(true)]);
let a = a.slice(1, 4);
let a = a.as_any().downcast_ref::<BooleanArray>().unwrap();
let c = not(&a).unwrap();

let expected = BooleanArray::from(vec![Some(false), Some(true), None, Some(false)]);

assert_eq!(c, expected);

Expected behavior
The code should not panic and return the correct result by using the bit_slice method.