Fix lint traceback for .cylc directories

[binggao1230]

Fixes #7344.

This prevents cylc lint from raising tracebacks for the issue reproduction where an empty flow.cylc exists alongside a directory named *.cylc.

Changes:

• Report matched non-files as lint errors instead of trying to open them.
• Treat empty lint inputs as having no lines to check.
• Add regression tests for empty files and .cylc directories.
• Add my name to the contributor list per CONTRIBUTING.md.

Verification:

• .venv/bin/python -m pytest tests/unit/scripts/test_lint.py
• .venv/bin/python -m flake8 cylc/flow/scripts/lint.py tests/unit/scripts/test_lint.py
• git diff --check
• Manual reproduction with touch flow.cylc, mkdir another.cylc, and cylc lint --exit-zero

[oliver-sanders]

Hi @gaoflow 👋, thanks for your interest in Cylc.

[oliver-sanders]

It might be safer to use if file.is_dir here as these files are sometimes symlinks.

[binggao1230]

Thanks, good point. I changed the guard to only special-case directories with file.is_dir(), so file symlinks and other openable paths are not rejected up front.

Local verification:

• .venv/bin/python -m pytest tests/unit/scripts/test_lint.py::test_check_cylc_file_reports_directory tests/unit/scripts/test_lint.py::test_check_cylc_file_empty -q -> 2 passed
• .venv/bin/python -m pytest tests/unit/scripts/test_lint.py -q -> 140 passed
• git diff --check -> passed