Fuel vapour int he tanks, in this case no 7 tank, is not as much of a problem as most people think in flight. The tanks are constantly vented using ram air taken in at the leading edge, then vented overboard at the trailing edge. Although there are several different ways this happens for the various tanks, the 7 Tank in question is vented by this simple method. This allows for the gas volume to increase when fuel is being emptied from the tank, and also to help get rid of any vapours.
Obviously on the ground this doesn't work...