And it's actually pretty clever from a performance point-of-view.

You always have to read the inode to determine whether the file is executable. The inode included the locations of the first "direct" blocks of the file's contents. So if the file is empty you know that immediately.

If the file even had a single comment line, it would mean having to do a disk seek to read it. So it would effectively double the disk I/O needed to run "/bin/true"