avoid newlines of env vars are empty

this is bugfix for previous commit to the template 29f06be3862945a01e46ab866671947e9ad698a1
i tested too. without any of the env vars, there's only one blank line:

    # Crontab for daily-usage-report managed by Chef. Changes will be overwritten.
    
    0 23 * * * appuser /srv/app/scripts/daily_report