The following applies to the Gen 1 Volt, and I suspect it also applies to the Gen 2.
Whenever you actually run out of gas, after you put new gas into the tank, the engine will start and run a short self-test. If all is ok, everything returns to normal and you continue on in the driving mode you choose. I once used the Torque Pro app to monitor my gas volume when parked on a level surface, and refilled when it read 0%, 0.0 gallons remaining. There was no engine self-test performed in this case, confirming what colchiro points out above. When the OBD readout reports zero gas, more fuel is actually remaining in the lines than if you had driven until the ICE stopped for lack of gas, so a few miles of driving still remains at this point.
Even if you run out of gas, a wee bit of fuel remains in the lines. When I ran out of gas in my 2012 Volt to end my first FMM and filled the tank, the engine ran a self-test, and my next FMM occurred ~363 days later. Our friend Ari also chooses to run out before adding the minimum amount that will end the FMM (~1.5 gallons), and his FMMs usually occur ~313 days later. That ~50 day difference in times to next FMM is because the "old" gas remaining in the lines after the tank is empty is enough to affect the overall "average age" of the ~1.5+ gallons now in his tank, lowering the next FMM date below the one year mark.