fix: preserve empty dict handler output

[chenlichao]

Summary

• Stop dropping an empty dict returned by a serverless handler from the normalized job result.
• Add regression coverage for local /runsync and /status simulation paths returning output: {}.

Why this helps

• Fixes Returning empty object from handler raises KeyError exception within framework #459: handlers that intentionally return {} should complete successfully instead of causing local FastAPI simulation to raise KeyError: 'output'.

Test Plan

• pytest tests/test_serverless/test_modules/test_fastapi.py -q --no-cov (8 passed)

Note: a targeted run without --no-cov also had all 8 tests pass, but failed the repository-wide coverage threshold because only one test file was selected.

[capy-ai]

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[promptless]

Promptless prepared a documentation update related to this change.

Triggered by runpod-python PR #535

Added a "Handler return values" section to the Serverless handler functions docs, explaining how a handler's return value maps to the job output field — dictionaries pass through directly, other types are wrapped, and an empty dictionary is a valid return that completes as COMPLETED with output: {}.

Review: Document Serverless handler return values