[C++] "may be used uninitialized" build error with gcc 16 in acero test utils

[adamreeve]

I get the following error building Arrow C++ with gcc 16.2.1 on x86_64 Fedora Linux 44, using the ninja-debug-python CMake preset:

/home/adam/dev/arrow/cpp/src/arrow/acero/test_util_internal.cc: In constructor ‘arrow::acero::{anonymous}::DummyNode::DummyNode(arrow::acero::ExecPlan*, arrow::acero::ExecNode::NodeVector, bool, arrow::acero::StartProducingFunc, arrow::acero::StopProducingFunc)’:
/home/adam/dev/arrow/cpp/src/arrow/acero/test_util_internal.cc:79:57: error: ‘<unknown>’ may be used uninitialized [-Werror=maybe-uninitialized]
   79 |       : ExecNode(plan, std::move(inputs), {}, (is_sink) ? nullptr : dummy_schema()),
      |                                               ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
/home/adam/dev/arrow/cpp/src/arrow/acero/test_util_internal.cc:124:27: note: by argument 1 of type ‘const arrow::acero::{anonymous}::DummyNode*’ to ‘std::shared_ptr<arrow::Schema> arrow::acero::{anonymous}::DummyNode::dummy_schema() const’ declared here
  124 |   std::shared_ptr<Schema> dummy_schema() const {
      |                           ^~~~~~~~~~~~
cc1plus: all warnings being treated as errors

dummy_schema() is a method of DummyNode but the DummyNode instance is not yet initialized when this is called.

[adamreeve]

Issue resolved by pull request 51279
#51279