Add regression coverage for None-returning orchestrations

[andystaples]

💡 Feature description

Add explicit regression tests for all supported None-return paths in orchestration replay:

• A sub-orchestrator whose completion event has Result=None.
• A top-level generator orchestrator that completes without an explicit return value.

Activity completion with Result=None is already covered by test_call_activity_with_none_return. The additional tests should verify that replay completes successfully, produces the expected None task value where applicable, and does not call JSON deserialization with None.

💭 Describe alternatives you've considered

Relying on the shared guard in TaskOrchestrationExecutor.parse_history_event and the existing activity test leaves the separate SUB_ORCHESTRATION_INSTANCE_COMPLETED branch without direct regression coverage. A top-level no-return test would also preserve the behavior that originally made issue #260 difficult to diagnose.

Additional context

Issue #260 reported the original failure. The root cause was isolated in #454 and fixed by #466, released in azure-functions-durable 1.2.9. Current code handles both task and sub-orchestration completion events with null results, but only the activity path has a focused regression test.

[andystaples]

Informational v2 status only; this does not close, reject, or otherwise decide this issue for v1. This regression request targets a v1 parse_history_event branch that does not exist in v2; v2's completion path explicitly handles None results without serializing them. There is not yet a dedicated v2 sub-orchestration-None regression test.