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GH-40997: [C++] Get null_bit_id according to are_cols_in_encoding_order in NullUpdateColumnToRow_avx2 (#40998)



### Rationale for this change
Recently, we find that the compare internal's avx2 function NullUpdateColumnToRowImp_avx2 lost the are_cols_in_encoding_order check when get null_bit_id. It may cause grouper's compare result wrong(are_cols_in_encoding_order = true in grouper).

### What changes are included in this PR?
Get `null_bit_id` according to `are_cols_in_encoding_order` in NullUpdateColumnToRow_avx2.

### Are there any user-facing changes?
No

Co-authored-by laotan332 <qinpengxiang@ outlook.com>
Co-authored-by ZhangHuiGui <2689496754@ qq.com>

* GitHub Issue: #40997

Lead-authored-by: ZhangHuiGui <hugo.zhang@openpie.com>
Co-authored-by: ZhangHuiGui <2689496754@qq.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>