re: "Can clients leave feedback if you ended the contract before any work was done or money exchanged?"
If YOU (as the freelancer) close the contract, then you MUST leave feedback.
But the client does NOT need to leave feedback. The client may or may not leave feedback. Once you close the contract, the client has a 14-day window during which he may leave feedback.
Because no money is exchanged, nothing will appear on your public profile page. No written feedback. No star average. No mention of the contract. The client's private feedback will NOT affect your JSS. Unless it is negative. Negative feedback left by a client for a zero-pay contract CAN have a negative impact on JSS.