Don't pass optional arguments to timeago()

Fixes the following error

```[Tue Sep 02 17:55:10 2014] [error] [client 127.0.0.1] PHP Notice:  Undefined variable: prospect in /var/www/Nagdash/phplib/timeago.php on line 40, referer: http://localhost/```

Passing in NULL as an argument causes the variable to be set to NULL
rather than it using the default. This was preventing " ago" from
being appended to the duration string.