Prevent a notice message from showing up.

library/Unl/Cas.php

@@ -80,6 +80,9 @@ class Unl_Cas
         	}
         	$this->_session = $_SESSION[__CLASS__];
         }
+        if (!isset($this->_session->ticket)) {
+            $this->_session->ticket = NULL;
+        }
     }
     
     /**