Goaccess: Display warning if access log is empty

[sentabi]

I setup goaccess but somehow i forgot to add access_log on my nginx server block. When run

goaccess /var/log/nginx/access.log
# OR 
goaccess -f /var/log/nginx/access.log

didn't complaint access.log file is empty. goaccess just quit after execute these command.

goaccess /var/log/nginx/access.log -o report.html

above command just echo /var/log/nginx/access.log without any warning.

I'd love if goaccess give warning if log is empty, so we know whats wrong :)

[allinurl]

If your log is empty, goaccess used to error out, now it will output an empty report. This change was part of issue #471.