Fix ALPN empty-string normalization mismatch in tests.test_security_compat_utils

[MichaelDecent]

Summary

tests.test_security_compat_utils has a failing ALPN normalization assertion in test_alpn_and_policy.

Reproduction

uv run python -m unittest -v tests.test_security_compat_utils

Current result

• Ran: 3 tests
• Failures: 1
• Failing test:
  • test_alpn_and_policy

Failure details

At:

• tests/test_security_compat_utils.py:17

Assertion:

self.assertIsNone(normalize_alpn(''))

Observed:

• normalize_alpn('') returns ''
• failure: AssertionError: '' is not None

Expected behavior

Clarify and align either:

1. implementation: empty ALPN normalizes to None, or
2. test expectation: empty ALPN normalizes to empty string.

Acceptance criteria

• Behavior contract for empty ALPN input is explicit.
• uv run python -m unittest -v tests.test_security_compat_utils passes.