B028: flag empty skip_file_prefixes tuples

[Sonike]

Fixes #510

Summary

• report B028 when warnings.warn receives an explicitly empty skip_file_prefixes=() tuple
• continue skipping calls whose prefixes are non-empty or cannot be determined statically
• document the exception and add regression coverage

An empty tuple does not cause warnings.warn to raise the effective stack level, so treating it like a populated skip_file_prefixes value suppresses a useful B028 diagnostic.

Tests

• .venv/bin/python -m pytest tests/test_bugbear.py -q (81 passed)
• .venv/bin/tox -e py314 (81 passed, 98% coverage)
• .venv/bin/pre-commit run --all-files (isort, black, flake8, rstcheck passed)

Limitations

Dynamic expressions remain intentionally conservative: if the value of skip_file_prefixes is not a literal empty tuple, B028 is skipped as before.

[cooperlees]

This is an improvement. Thanks!