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GH-40731: [C++][Parquet] Minor enhancement code of encryption (#40732)



### Rationale for this change

* Applying more `std::move` in encryption
* Trying to use `NULLPTR` rather than `NULL`.

### What changes are included in this PR?

Applying more `std::move` in encryption

### Are these changes tested?

No need

### Are there any user-facing changes?

No

* GitHub Issue: #40731

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