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GH-35635: [C++][CI] Preserve root when ignoring host on PathFromUriHelper to fix HDFS tests (#36063)

### Rationale for this change

As discussed on the issue the HDFS filesystem should be returning `/` if the entire path is `/`

### What changes are included in this PR?

Change behavior of helper `PathFromUriHelper` when ignoring host to preserve `/`

### Are these changes tested?

On CI and archery.

### Are there any user-facing changes?

No
* Closes: #35635

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>