Skip hpopt reporting when no result was provided - #3082

What does this PR do?

Fixes #2358.

Exiting a hyperparameter-optimization context without calling report_result() reports the initial {"objective": 0.0} placeholder. This raises KeyError with a custom objective key, including evaluation-only runs, or emits a result that was never measured with the default key.

Initialize the pending result to None, which the existing exit guard already handles. Explicitly reported results, including zero, still go through the selected reporter or generic fallback.

Validation:

• New regression cases cover no result with custom/default objective keys and hpopt enabled/disabled. Before the fix: 4 failed, 5 passed (2 KeyErrors and 2 unwanted zero reports).
• pytest tests/unittests/test_hpopt.py --doctest-modules speechbrain/utils/hpopt.py -q: 18 passed. This also checks explicit zero results, last-result selection and context cleanup using the real generic reporter.
• pre-commit run -a and git diff --check passed.

No model downloads or training runs were needed. The full suite and a live Oríon experiment were not run. No new dependencies or API changes.

Implemented and tested with OpenAI Codex, with independent read-only review by another Codex agent.

PR review