Putting it very simply, if you switch your microwave on with NOTHING in it then you will get exactly the same results as if you put something metal in it. The arcing will be inside the workings of the microwave oven rather than from the metal object to the sides of the oven. You probably won't see this internal arcing and it will only last as long as to fry the innards and stop the oven from working.
However, if you have some food or liquid inside the microwave then the energy emitted by the microwave is absorbed by the food and no harm is done. The stainless steel sides do not even enter the process.
Hope you still have a microwave left!!