set_where_group fails when no where arguments added

I recently made use of set_where_group to make a group of where arguments be grouped by "OR" instead of "AND" - however - if there is no where arguments added it fails on finding out the number of the new group because $this->$where doesn't contain any key to base the new group number on.

Maybe I have done my implementation wrong so that this would never be the case - but if I haven't then this is a bug which I've done a patch for fixing.