Fix error in CiviCRM 4.5 (raised as a worrying SECURITY ALERT) when ...

...accessing Manage Groups page.

After upgrading from 4.4.6 to 4.5 and accesing the groups page I got this popup:

> "SECURITY ALERT: Ajax requests can only be issued by javascript clients, eg. CRM.api3"

This turned out to be to do with the way the API call was parsing an empty group ID string from the request.