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 ...
Did you possibly try using https://www.easycron.com/ (as described https://docs.civicrm.org/sysadmin/en/latest/setup/jobs/#webcron) and seeing if that will work. Like this you can see if the problem is with your civi instalation or your web host.
I prefer the wp-cli method to all other methods. Can this work at your host? https://docs.civicrm.org/sysadmin/en/latest/setup/jobs/#wp-cli
If you do not have Command line access, but the host provides a cron interface and wp-cli you can create a shell script to call:
/path/to/cli/php /path/to/wp --user=<cron user> --url=https://<...