Enable Empty Response Retry with Channel Switching

[puyujian]

Summary

Introduce an explicit control to treat empty upstream responses as retriable errors, enabling automatic retries and channel switching when an upstream channel returns no content.

Details

• Add EmptyResponseRetryEnabled to global retry settings (backend).
• Add ShouldTreatEmptyResponseAsError() helper to centralize logic.
• Wire empty-response retry into the relay flow so empty upstream replies trigger retry and optional channel switching.
• Extend the front-end settings UI with a new toggle to enable empty-response retry (requires Auto Retry to be on).
• Update internationalization strings for empty-response retry UI.
• Adjust related tests and logging to reflect new behavior.