Nothing, unless you are later on subpoena'ed as a witness. You weren't involved in the collision so there is no insurance claim, and you weren't charged with anything so there is nothing to worry about in that regard.
It's possible that at some later date, that your presence may be subpoena'ed in court as a witness to the collision, if someone takes their charge to court. If that happens, just be at the court at the time and date that they tell you to be there, do what the court tells you to do, and answer the questions that you are asked.