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

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

**Reporter**: [Wes McKinney](https://issues.apache.org/jira/browse/ARROW-16845) / @wesm
#### Related issues:
- [[C++] Improve array expression and kernel evaluation performance on small inputs](https://github.com/apache/arrow/issues/20277) (relates to)

_{**Note**: *This issue was originally created as [ARROW-16845](https://issues.apache.org/jira/browse/ARROW-16845). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by tracing the C++ ArraySpan::IsNull and IsValid implementations and read the follow-up context from ARROW-16756. Verify how union types represent their first buffer, then add or update coverage showing that validity checks no longer treat that buffer as a validity bitmap.

Written by the indexing model from the issue text.