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GH-34361: [C++] Fix the handling of logical nulls for types without bitmaps like Unions and Run-End Encoded (#34408)

Bonus: add `ArrayData::IsValid()` to make it consistent with `Array` and `ArraySpan`.

### Rationale for this change

This is the proposed fix to #34361 plus the addition of more APIs dealing with validity/nullity.

### What changes are included in this PR?

This PR changes the behavior of `IsNull` and `IsValid` in `Array`, `ArrayData`, and `ArraySpan`.

It preserves the behavior of `MayHaveNulls`, `GetNullCount` and introduces new member functions to `Array`, `ArrayData`, and `ArraySpan`:

 - `bool HasValidityBitmap() const`
 - `bool MayHaveLogicalNulls() const`
 - `int64_t ComputeLogicalNullCount() const`

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes. See above.

Breakage with these changes can only happen if users rely on `IsNull(i)` always returning `true` for union types, but we have users reporting that the current behavior or broken #34315. This is why the behavior of `IsNull` and `IsValid` is changing.,

**This PR contains a "Critical Fix".**
* Closes: #34361

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>