Preserve empty responses in prompt normalizer batches

[biefan]

Summary

• turn None / empty target responses into explicit empty response messages in PromptNormalizer.send_prompt_async()
• preserve one result per request in send_prompt_batch_to_target_async() instead of filtering empty responses away
• add regression coverage for single-request empty responses and batch response alignment

Problem

PromptNormalizer.send_prompt_async() currently returns None when a target returns no messages, and the batch helper filters those None entries out. That breaks the one-request-to-one-response contract for batch callers and can misalign downstream consumers that pair responses with their original prompt list.

Validation

• .venv/bin/pytest tests/unit/prompt_normalizer tests/unit/executor/promptgen/fuzzer -q
• .venv/bin/ruff check pyrit/prompt_normalizer/prompt_normalizer.py tests/unit/prompt_normalizer/test_prompt_normalizer.py