GH-48965: [Python][C++] Compare unique_ptr for CFlightResult or CFlightInfo to nullptr instead of NULL

[raulcd]

Rationale for this change

Cython built code is currently failing to compile on free threaded wheels due to:

/arrow/python/build/temp.linux-x86_64-cpython-313t/_flight.cpp: In function ‘PyObject* __pyx_gb_7pyarrow_7_flight_12FlightClient_9do_action_2generator2(__pyx_CoroutineObject*, PyThreadState*, PyObject*)’:
/arrow/python/build/temp.linux-x86_64-cpython-313t/_flight.cpp:43068:110: error: call of overloaded ‘unique_ptr(NULL)’ is ambiguous
43068 |           __pyx_t_3 = (__pyx_cur_scope->__pyx_v_result->result == ((std::unique_ptr< arrow::flight::Result> )NULL));
      |                            

What changes are included in this PR?

Update comparing unique_ptr[CFlightResult] and unique_ptr[CFlightInfo] from NULL to nullptr.

Are these changes tested?

Yes via archery.

Are there any user-facing changes?

No

• GitHub Issue: [Python][C++] free-threaded wheels for Python 3.13 are failing to build flight #48965

[github-actions]

⚠️ GitHub issue #48965 has been automatically assigned in GitHub to PR creator.

[raulcd]

@github-actions crossbow submit wheel-manylinux-2-28-cp313-cp313t-*

[github-actions]

Revision: a96a120

Submitted crossbow builds: ursacomputing/crossbow @ actions-821088a53e

Task	Status
wheel-manylinux-2-28-cp313-cp313t-amd64
wheel-manylinux-2-28-cp313-cp313t-arm64

[raulcd]

I am quite confused that this just started failing on these two nightlies and the last handful of days:

• wheel-manylinux-2-28-cp313-cp313t-amd64
• wheel-manylinux-2-28-cp313-cp313t-arm64

this seems to fix the problem but I have no idea why it didn't fail on other places or previously. @pitrou any thoughts?

[raulcd]

I validated the cython version used was the same for both the last successful build and the first failure. Based on the info from:
https://s3.amazonaws.com/arrow-data/index.html

Last successful build: https://github.com/ursacomputing/crossbow/actions/runs/21164890755/job/60867111904
First failure: https://github.com/ursacomputing/crossbow/actions/runs/21241731659/job/61121044657