SlotRef::tuple_is_nullable_ uninitialised for struct children

In SlotRef::Init, tuple_is_nullable_ is only assigned a value if the SlotRef is not within a struct:

if (!slot_desc_->parent()->isTupleOfStructSlot()) {
      tuple_is_nullable_ = row_desc.TupleIsNullable(tuple_idx_);
}

https://github.com/apache/impala/blob/fefb9f24be1f99ac0077a8d6ef00834d8e90ef45/be/src/exprs/slot-ref.cc#L103

Otherwise tuple_is_nullable_ remains uninitialised, leading to undefined behaviour when it is read.