An empty fuel tank is more likely to catch fire than a full/half-full one. Not sure of the modeling in the game for fuel tanks, or if the self-sealing tanks some birds have work in this game, but I do know that the incendiary rounds would be the only type that would cause a fire.
There is no such thing as an empty fuel tank on an aircraft. There is always some amount of "unusable" fuel that will remain in the tank. Certainly enough to produce flammable vapors. A partially empty tank with lots of vapors would be more likely to "explode" if the vapors in the tank are ignited, but I can't see where an empty tank would be more likely to burn than a tank with lots of fuel in it -- the full tank has much more fuel for the fire, will have a more productive leak if punctured.
A lot of fires on aircraft can be caused by the aircraft itself once the fuel tank is compromised. Ever notice in the gun camera films how many wing fires you see on B-17s? There are 3 fuel tanks holding 850 gallons in each wing aft of the engines on the Fortress. Behind each engine and aft of the fuel tanks is the exhaust outlet/wastegate/turbocharger outlet.
Take a peek at this video showing the exhaust flame. FYI, this is at a low cruise power setting of 28" MP, 1800 RPM in Auto-lean. I'm guessing that could easily start a fire if the fuel tank was leaking. <G>
