arrow

GH-43594: [C++] Remove std::optional from arrow::ArrayStatistics::is_{min,max}_exact (#43595)

### Rationale for this change

We don't need "unknown" state. If they aren't set, we can process they are not exact.

### What changes are included in this PR?

Remove `std::optional` from `arrow::ArrayStatistics::is_{min,max}_exact`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #43594

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>