arrow

GH-41464: [Python] Fix StructArray.sort() for by=None (#41495)

### Rationale for this change
Closes issue https://github.com/apache/arrow/issues/41464. Fix `StructArray.sort` method's `by` param to work in the case of `by=None` which was documented to mean sort by all fields (the default), but would raise an exception.

### What changes are included in this PR?
* Add a unit test with by=None in `test_struct_array_sort` that fails on main
* Fix the sort method

### Are these changes tested?
yes

### Are there any user-facing changes?
yes
* GitHub Issue: #41464

Authored-by: a-reich <asafspades@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>