"Zero, since it doesn't actually provide thrust."
That in itself would violate Newton's Laws of Motion, wouldn't it? Specifically, that would be Newton's Third Law, for every action, there is an equal and opposing reaction. If you're emitting something, even inside of a closed cavity, THERE MUST BE INITIAL THRUST/PUSH OFF OF THE EMITTING MATERIAL, NO MATTER HOW SMALL, EITHER POSITIVE OR NEGATIVE.
Back to school with you.