Optimize NULL handling in `array_slice`

[neilconway]

Is your feature request related to a problem or challenge?

array_slice does a per-row NULL check (in four different arrays); it would be faster to take the union of the four input NULL buffers via NullBuffer::union.

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response

[neilconway]

take