arrow

GH-43733: [C++] Fix Scalar boolean handling in row encoder (#43734)



### Rationale for this change

See https://github.com/apache/arrow/issues/43733

### What changes are included in this PR?

Separate Null and Valid handling when BooleanKeyEncoder::Encode meets a Null

This patch also does a migration:
* row_encoder.cc -> row_encoder_internal.cc
* move row_encoder_internal{.cc|.h} from `compute/kernel` to `compute/row`

### Are these changes tested?

Yes

### Are there any user-facing changes?

No

* GitHub Issue: #43733

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>