Fix $data warning

Commit 23b7ee23 optimizes CRM_Civirules_Utils_PreData::customPre() but in a way that can cause an "undefined variable" warning. I'm moving the declaration of $data so it's always initialized.

I'm a bit confused by this optimization though. If self::$preData[$entity][$entityID][$eventID]) exists, should we set $data to its value?