Opt-out sometimes causes a warning to be displayed

When clicking an opt-out link in an email, e.g.

https://www.example.com/index.php?q=civicrm/mailing/optout&reset=1&jid=115&qid=137&h=ae8d24c3f12687b3d9dfda15b7af9840e329370f

and then clicking the 'Optout' button, the action is performed successfully. However, a warning is produced:

warning: Invalid argument supplied for foreach() in /home/example/public_html/modules/civicrm/CRM/Mailing/Event/BAO/Unsubscribe.php on line 265.

Looking through the code, there are multiple calls to CRM_Mailing_Event_BAO_Unsubscribe::send_unsub_response() with the $groups parameter set to NULL. This patch will skip the problematic foreach loop if $groups is not an array, preventing the warning.