Yes, there's a tiny little cavity, surrounded by curing concrete - concrete that will continue to cure for a couple of hundred years. As the sealed remains decay, the concrete will wick out the water over a long period of time. The concrete doesn't and won't collapse upon itself.
During large concrete pours, pains are taken to ensure no foreign debris enters the mix, like a scrap of wood, for instance.
You may be right, but I do know it is not an ideal situation for the integrity of a structure to have something so large as a dead body buried within its concrete.