It’s possible that either the glass or the meatball allowed charge to build up to the point of arcing. It’s also possible that there is something wrong with the microwave. Try putting a cup of water in the microwave and running it for about 30 seconds. If there is no arcing then the microwave itself is probably fine. If you continue to have arcing then replace the microwave.
As for running a microwave empty, that’s a bit harsh on the magnetron (the thing that generates the radio waves). Back in the 70s it was very easy to break the magnetron doing this. These days magnetrons are a bit more rugged, but I still would advise against running the microwave completely empty (hence the cup of water above to test it).
You have to be a bit careful with metal in a microwave. Large pieces of metal can reflect the radio waves back into the magnetron (which again, is a bit harsh on it). The radio waves also induce eddy currents into the metal, causing it to heat up. If you have really thin bits of metal, the metal can actually heat up to the point where it burns or melts. Steel wool for example makes a pretty impressive display inside your microwave (again, magnetron warning, don’t do this it home, etc). A thicker piece of metal like a spoon will just get hot, possibly so hot that it will give you a nasty burn when you touch it. Charge will build up on pointy bits of metal, which can lead to arcing.