you can’t dump to empty tanks. There are standpipes in the fuel tanks. I think dumping is limited to about an hour endurance,
Standpipe levels have come up before - just for some precision this from a current FCOM.
777-200 .."At least 5200 kilograms of fuel remains in each main tank after jettison is complete."
777-300 .." At least 3800 kilograms of fuel remain in each tank after jettison is complete..."