Reject explicit empty objectives in batch scoring

[biefan]

Summary

• distinguish omitted objectives from an explicit empty sequence in batch scoring entry points
• reject objectives=[] when scoring non-empty prompt or image batches
• add regression coverage for both prompt and image batch scoring paths

Problem

score_prompts_batch_async() treats objectives=[] the same as objectives=None, replacing it with default empty-string objectives and continuing into the scoring pipeline. score_image_batch_async() has the same issue. This skips the documented length validation and can trigger unrelated downstream failures instead of failing fast.

Validation

• .venv/bin/pytest tests/unit/score -q
• .venv/bin/ruff check pyrit/score/scorer.py tests/unit/score/test_scorer.py