S3Guard in auth mode doesn't raise AccessDeniedException on read of 0-byte file

This falls out of auth mode knowing the length of a file and skipping any S3 checks: it is not an error to read a 0-byte file, regardless of the readability of the file.

• there's no check in open()
• and read() just returns -1.

I don't see if that is fixable, or if it merits. Maybe just include in the release notes