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If the page has not changed since the date given in the request, your server will return a 304-Not Modified response, and zero bytes of page content. Some robots use HEAD requests to do this as well.
The purpose is to keep your pages' listings current without unnecessarily downloading pages that have not been updated since the robot's last visit.
Either way, the indication that CGETs or HEAD requests are the cause of these zero-byte responses is that the requested pages are those that you have not changed since the robot's last visit.
Jim