fix(benchmark): Fix `TestResult.fail_reason` assignment condition

The condition must be the same as for `success`, because otherwise it causes a crash when `call.excinfo` evaluates to `False` but is not `None`.

benchmark/agbenchmark/reports/reports.py

@@ -90,7 +90,7 @@ def add_test_result_to_report(
         TestResult(
             success=call.excinfo is None,
             run_time=f"{str(round(call.duration, 3))} seconds",
-            fail_reason=str(call.excinfo.value) if call.excinfo else None,
+            fail_reason=None if call.excinfo is None else str(call.excinfo.value),
             reached_cutoff=user_properties.get("timed_out", False),
             n_steps=user_properties.get("n_steps"),
             cost=user_properties.get("agent_task_cost"),