[C++] ArraySpan::IsNull/IsValid implementations are incorrect for union types

[asfimport]

Because the first buffer is not a validity bitmap. Follow up work from ARROW-16756

Reporter: Wes McKinney / @wesm

Related issues:

• [C++] Improve array expression and kernel evaluation performance on small inputs (relates to)

_{Note: This issue was originally created as ARROW-16845. Please see the migration documentation for further details.}