Fix Github test failure.
Fix Github test failure.

This is similar with https://source.corp.google.com/piper///depot/google3/third_party/clif/testing/lambda_expressions.h;l=46. Without defining a default constructor explicitly, when running under the Github environment, PyCLIF generates `::absl::optional` that wraps the return value :https://source.corp.google.com/piper///depot/google3/third_party/clif/python/gen.py;l=607. This won't happen when running the test under internal environment.

PiperOrigin-RevId: 447083196

google/clif,google/clif,google/clif