Zero bias really just means no external bias. If the two electrodes are different, they will have different Fermi level and thus an internal bias still exists. So there is no special treatment of this case, the calculations proceed as for any device, except that the crucial in which the two Fermi levels are brought into alignment (no bias!) results in a shift of the bands in one electrode, and a current will flow from occupied to unoccupied bands (but backwards, since current is opposite electron flow). That is why left-right and right-left currents might differ in this case.
