Validate explicit empty field overrides in attack executor

[biefan]

Summary

• treat field_overrides=[] as an explicit value that still participates in length validation
• apply the same fix to both execute_attack_async() and execute_attack_from_seed_groups_async()
• add regression tests for the objective and seed-group execution paths

Problem

AttackExecutor currently uses truthiness checks for field_overrides. That means an explicit empty list is treated the same as None, so the documented length validation is skipped instead of raising a mismatch error for non-empty objectives or seed groups.

Validation

• .venv/bin/pytest tests/unit/executor/attack/core/test_attack_executor.py -q
• .venv/bin/ruff check pyrit/executor/attack/core/attack_executor.py tests/unit/executor/attack/core/test_attack_executor.py