By empty, do you mean empty and depressurized?
I suppose the 2.1 suggestion above is technically correct as it will be a gas.
Surely if you depressurize the tanks and flush them with N2 then there is no DG risk whatsover anyway. That is probably the safest option.
I was probably wrong with my Class 3 guess above, as I've not read the DG manual for years, can someone just remind me, is the class of liquid or gas ie 3 or 2 considered under ambient conditions, or under the conditions in the vessel it is stored in?