Well, I would prefer it be full of water, but being full of fuel is safer than being full of fuel fumes. You can stick one of those wand lighters into gasoline and click it all day without anything happening. No fumes, no oxygen, no fire. But put it into an empty gasoline can and try the same thing. Well, don't really try it, or you'll go boom.
I grew up in a body shop. One day my dad was using a cutting torch, and blew some sparks/slag across the shop and one went down the spout of an "empty" gas can. The explosion bulged the bottom of the can out so hard if flew straight up to the ceiling and smashed a flourescent light fixture.