Ensure that the "creating default object from empty value" warning is
always thrown. Previously some cases were missing the warning, in
particular those going through FETCH_OBJ_W rather than a dedicated
opcode (like ASSIGN_OBJ).
One slightly unfortunate side-effect of this change is that something
like $a->b->c = 'd' will now generate two warnings rather than one
when $a is null (one for property b, one for property c).
