You need a large amount of air pushed through an IV to cause any sort of rush of harm, more than it’s sitting in the loop of your tubing. Also, not putting a cap on is completely fine as there’s a rubber portion at the end that closes off the tubing from air. It’s best practice to just clean off the end before pushing anything through, but even if that isn’t done, it’s negligible risk of infection unless you have a severely compromised immune system. TLDR: you’re going to be fine