GH-45739: [C++][Python] Fix crash when calling hash_pivot_wider without options (#45740)

### Rationale for this change

Calling `hash_pivot_wider` without options is not very useful (it defaults to `key_names=[]`) but at least it shouldn't crash.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Just a bugfix.
* GitHub Issue: #45739

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

Author: pitrou

cpp/src/arrow/compute/kernels/hash_aggregate_pivot.cc

@@ -452,9 +452,11 @@ const FunctionDoc hash_pivot_doc{
 }  // namespace
 
 void RegisterHashAggregatePivot(FunctionRegistry* registry) {
+  static const auto default_pivot_options = PivotWiderOptions::Defaults();
+
   {
-    auto func = std::make_shared<HashAggregateFunction>("hash_pivot_wider",
-                                                        Arity::Ternary(), hash_pivot_doc);
+    auto func = std::make_shared<HashAggregateFunction>(
+        "hash_pivot_wider", Arity::Ternary(), hash_pivot_doc, &default_pivot_options);
     for (auto key_type : BaseBinaryTypes()) {
       // Anything that scatter() (i.e. take()) accepts can be passed as values
       auto sig = KernelSignature::Make(

python/pyarrow/tests/test_table.py

@@ -2975,6 +2975,32 @@ def test_table_group_by_first():
     assert result.equals(expected)
 
 
+@pytest.mark.acero
+def test_table_group_by_pivot_wider():
+    table = pa.table({'group': [1, 2, 3, 1, 2, 3],
+                      'key': ['h', 'h', 'h', 'w', 'w', 'w'],
+                      'value': [10, 20, 30, 40, 50, 60]})
+
+    with pytest.raises(ValueError, match='accepts 3 arguments but 2 passed'):
+        table.group_by("group").aggregate([("key", "pivot_wider")])
+
+    # GH-45739: calling hash_pivot_wider without options shouldn't crash
+    # (even though it's not very useful as key_names=[])
+    result = table.group_by("group").aggregate([(("key", "value"), "pivot_wider")])
+    expected = pa.table({'group': [1, 2, 3],
+                         'key_value_pivot_wider': [{}, {}, {}]})
+    assert result.equals(expected)
+
+    options = pc.PivotWiderOptions(key_names=('h', 'w'))
+    result = table.group_by("group").aggregate(
+        [(("key", "value"), "pivot_wider", options)])
+    expected = pa.table(
+        {'group': [1, 2, 3],
+         'key_value_pivot_wider': [
+             {'h': 10, 'w': 40}, {'h': 20, 'w': 50}, {'h': 30, 'w': 60}]})
+    assert result.equals(expected)
+
+
 def test_table_to_recordbatchreader():
     table = pa.Table.from_pydict({'x': [1, 2, 3]})
     reader = table.to_reader()