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GH-35359: [C++] FixedSizeListArray.flatten() errors if all elements are null (#35674)



### Rationale for this change

### What changes are included in this PR?
Added a check inside of `FlattenListArray` before the call to `Concatenate` so that it will return an empty array of the appropriate type if there are no non null fragments to return. This avoids receiving the error from concatenate that you must pass at least one array to it.

### Are these changes tested?
Unit tests are added.

* Closes: #35359

Authored-by: Matt Topol <zotthewizard@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>