`ruff server` should check `find_user_settings_toml()` if no file configuration exists

When a workspace folder doesn't have any configuration available, `ruff server` should try `find_user_settings_toml()` as a fallback. This would try reading `~/.config/ruff/.ruff.toml`, `~/.config/ruff/ruff.toml` and `~/.config/ruff/pyproject.toml` on Linux.