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GH-33971: [C++] Fix AdaptiveIntBuilder to always populate data buffer (#33994)

If the AdaptiveIntBuilder was empty it would yield a null for the values buffer.  The byte_size.h utilities were not expecting this which led to the error reported in the issue.

Example:

```
>>> pa.array([], type=pa.dictionary(pa.int32(), pa.string())).buffers()
[None, None]
```
* Closes: #33971

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>