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GH-34696: [C++] Check REE arrays have no null buffer in Validate() (#34697)

### Rationale for this change

See #34696

### What changes are included in this PR?

Adds another check. Also removes DCHECKs from `SetData()`, since they are redundant with the `Validate()` checks and I don't think we do that for other arrays. Having them present made it harder to get to the root cause of this error, because `SetData()` is called on the array when validating a record batch.

### Are these changes tested?

* [x] Add tests

### Are there any user-facing changes?

**This PR contains a "Critical Fix".** Without this, sending a REE array with an invalid null buffer through IPC to the C++ implementation will cause it to crash if the array is validated.
* Closes: #34696

Authored-by: Will Jones <willjones127@gmail.com>
Signed-off-by: Will Jones <willjones127@gmail.com>