Fix empty slice

[mikasenghaas]

Description

If vf-eval crashes before any event loop lag is measured, the printing of results fails because numpy cannot compute statistics over an empty list. This PR fixes this by guarding the printing, i.e. only print if >= 1 measurement has been made.

Before

After

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

---

Note

Prevents a crash when printing evaluation results if no event loop lag measurements were collected.

• In verifiers/utils/eval_utils.py, print_results now prints the "Performance" section only when event_loop_lags is non-empty (if event_loop_lags:), avoiding numpy empty-slice statistics on an empty list.

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