Add PR comment renderer fallback tests

[Scottcjn]

Summary

• add coverage for the empty-report fallback row in render_pr_comment()
• add coverage to ensure non-dict report rows are ignored without leaking into the output
• add coverage for the missing repro_command fallback to `n/a`

Testing

• pytest tests/unit/test_report_renderers.py -q
• git diff --check

Closes #40.

[awamonem]

Looks good. This covers the fallback cases requested in #40 and the targeted test file passes. Approving.