Fix empty-dict consistency in ground-truth export

Devin Review (post-merge on PR #6) flagged that the `final_result`
assignment used a truthiness check (`if job_row.result`) while
`build_initial_result` used an identity check (`is None`). For a
job whose result is an empty dict (`{}`), the emitted
`GroundTruthSample` ended up with `initial_result={}` but
`final_result=None` — logically inconsistent.

Switch the final-result assignment to the same `is None` check so
both fields agree. Added `test_empty_dict_result_stays_consistent`
to lock the invariant in, and fixed the test helper so callers can
pass `{}` without the helper's `or` fallback replacing it.

Co-Authored-By: adrian kuman firmansah <adriancuman@gmail.com>

src/ocr_sprint/ground_truth/service.py

@@ -171,7 +171,10 @@ def iter_ground_truth_samples(
             reviewed_at=job_row.reviewed_at,
             created_at=job_row.created_at,
             initial_result=initial,
-            final_result=copy.deepcopy(job_row.result) if job_row.result else None,
+            # Use an ``is None`` check to stay consistent with
+            # ``build_initial_result``; otherwise an empty-dict result
+            # would produce ``initial_result={}`` but ``final_result=None``.
+            final_result=(copy.deepcopy(job_row.result) if job_row.result is not None else None),
             corrections=[
                 GroundTruthCorrection(
                     field_path=c.field_path,