If nothing ever eats a shark, what will eventually happen to the energy that is stored in its body?

Energy cannot be destroyed but can change from one sate to another. Usually in a food chain animals eaten and the energy is passed from one animal to the next. In the case of larger animals the energy is only made available is they pass away or lose an appendage such as a tooth or antlers, that other animals can feed on.

If a shark is not preyed on and remains in the ecosystem, it's energy will not be cycled but this animal will get larger and larger.

Should a shark pass away and nothing feeds on it there are two things that will happen:

Energy will be stored and the animal will eventually become volatile or slowly disintegrate and the energy will be released from the atoms and molecules when the bonds are eventually broken by chemical reactions brought on by the environment (sun. wind, currents, etc).
The animal will eventually solidify and may turn into stone. The energy will be locked into the animal and will be encased and become a part of the substrate

It is not likely that a shark will keep it's energy and not be eaten by other animals after death but on the chance that it does it may slowly disintegrate or become solidified.