A tank is actually safer with a few ounces of gas in it than one that's been sitting empty for awhile. The gas fumes will purge the air out of the tank, and the resulting too-rich mixture can't possibly burn due to lack of oxygen. If you want to be absolutely sure there's no possibility of a burn, stick a piece of garden hose into the tailpipe of another vehicle, put the other end into the empty tank, and fill it with exhaust gas. I've done repairs on leaky fuel tanks with a torch and a brass rod after filling the tank with exhaust fumes. All the oxygen gets displaced by the exhaust gas, which is mostly CO2 and water vapor.
Jerry
