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GH-41016: [C++] Fix null count check in BooleanArray.true_count() (#41070)

### Rationale for this change

Loading the `null_count` attribute doesn't take into account the possible value of -1, leading to a code path where the validity buffer is accessed, but which is not necessarily present in that case.

### What changes are included in this PR?

Use `data->MayHaveNulls()` instead of `data->null_count.load()`

### Are these changes tested?

Yes

* GitHub Issue: #41016

Authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>