blank emails from cron after upgrade Wordpress
This turned out to be something adding a blank line in the cron output (see comments above). Since it's not clear what is adding it or how to track it down, a workaround is to adjust the cron command slightly:

your-cron-command | grep "."

which will remove all blank lines from the output, so if the output is just a newline then after the grep the output will be completely empty and not send an email as normal. In some hosting control panels you might need to backslash the pipe character, or create a wrapper script and have cron call that script.