[COOK-3147] - Only set LOGFILE and -L when log_location actually set in attributes so ...

...that we can properly fall back to STDOUT when it is not

Mind you I'm not super sure why anyone would want to do this in a service script, but the recipe does seem designed in a way that makes it look like that's the intention, so the templates should be made to support it.

As an aside, we might want to start setting `node['chef_client']['log_file']` by default if we do this, since a lot of people probably intend to use /var/log/chef/client.log but never bothered to configure the attribute since the log ends up there on all machines anyway (due to `LOGFILE=node['chef_client']['log_dir']/client.log` being in /etc/${conf_dir}/chef-client on all platforms)