fix: preserve empty dict handler output

[chenlichao]

Summary

• Preserve {} returned by serverless handlers as a valid output payload.
• Add regression coverage for run_job() and the local runsync simulation path.

Why this helps

• Fixes Returning empty object from handler raises KeyError exception within framework #459, where a handler returning an empty dict caused _sim_runsync() to raise KeyError: 'output' while building the completed response.

Test Plan

• pytest tests/test_serverless/test_modules/test_job.py::TestJob::test_run_job_preserves_empty_dict_output tests/test_serverless/test_modules/test_fastapi.py::TestFastAPI::test_runsync -q --no-cov
• git diff --check

Note: the same targeted pytest command without --no-cov ran both tests successfully, but the repository-wide coverage gate failed because only two focused tests were selected.

[capy-ai]

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