fix(webdav): support blank-password nextcloud shares

[Orinks]

Summary

• Force WebDAV Basic auth through the requests session when a username is provided, including blank-password shares.
• Treat trailing-slash WebDAV fallback paths as directories when SabreDAV/Nextcloud omits explicit directory metadata from info responses.
• Add regression coverage for blank-password WebDAV auth and sparse directory info responses.

Test Plan

• uv run python -m ruff check src\portkeydrop\protocols.py tests\test_protocols.py
• uv run python -m ruff format --check src\portkeydrop\protocols.py tests\test_protocols.py
• uv run python -m pytest tests/test_protocols.py -k webdav
• Live WebDAV probe against the reported Nextcloud public share with blank password and three subdirectories
• uv run python -m pytest