Fixed zfs_notify_email for programs like sendmail

zfs_notify_email will now include an empty line separating the header
from the body of the email in case the subject is not provided via a
command line argument. This is necessary for programs like sendmail to
function correctly (everything up to the first empty line is interpreted
as header, which previously resulted in either missing message parts or
unsent emails)

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Felix Schmidt <felixschmidt20@aol.com>
Closed #17238