fix: unit test error when coverage wasn't specified

[terrykong]

What does this PR do ?

@YUki-666 reported pytest failing when not using tests/run_unit.sh and bare pytest

ERROR tests/unit/models/generation/test_vllm_generation.py::test_vllm_generate_text - UnboundLocalError: cannot access local variable 'cov_controller' where it is not associated with a value

This just fixes it so it doesn't error and leaves a message to turn it on in the unit_results.json

{
  "exit_status": 0,
  "git_commit": "084d6facd38f5a14c54adff11e32031be28451db",
  "start_time": "2025-03-26 09:24:12",
  "metrics": {},
  "gpu_types": [
    "NVIDIA H100 80GB HBM3"
  ],
  "coverage": "[n/a] run with --cov=nemo_reinforcer"
}

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