A tank full of gas won't explode if punctured, but there might be a fire if it leaks near something hot enough. An empty tank will explode with just the slightest spark. A half tank just depends on where it gets punctured, but with gravity and all, it's likely to be the bottom half that gets punctured.
Cars use in-tank pumps turned by brushed electric motors. Why don't they blow up when empty?
Even in an empty fuel tank the vapors are way to rich to ignite. You might get a good flash if you introduce air and ignition at the same time but odds are you won't get any sort of sustained fire.
