Ensure win_friendly_path doesn't error out when ::File::ALT_SEPARATOR is nil

This can happen when using the win_friendly_path method on a Linux box. This might happen when writing ChefSpec tests on a non-Windows host.
Merge pull request #78 from daptiv/COOK-4620

[COOK-4620] Ensure win_friendly_path doesn't error out when ALT_SEPARATOR is nil