arrow

GH-37851: [C++] IPC: ArrayLoader style enhancement (#37872)



### Rationale for this change

Enhance the style of `arrow::ipc::ArrayLoader`'s `SkipField`

### What changes are included in this PR?

Set `out_` to `nullptr` when `SkipField` is called.

### Are these changes tested?

No

### Are there any user-facing changes?

No

* Closes: #37851

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>