check_rabbitmq_overview: gently deal with idle queues

 I ran into a little issue with check_rabbitmq_overview: if all queues are idle, the rabbitmq api doesn't return an array for 'queue_totals'. Resulting in this error:
Not a HASH reference at /usr/local/nagios/libexec/check_rabbitmq_overview line 125.

If I view the returned json from the api/overview site:
          'rabbitmq_version' => '3.1.5',
          'erlang_version' => 'R14B04',
          'queue_totals' => []
        };
'queue_totals' is empty which causes the error, I added a check for this case. With this patch check_rabbitmq_overview returns:
./check_rabbitmq_overview --host=xxhost
RABBITMQ_OVERVIEW OK