Ha ha! You actually noticed that, eh? The "clack-clack" from pumping the slide under the barrel is actually the sound of a shell (like a bullet) being loaded into the firing chamber of the gun. And yes, you are right: It makes NO sense for the person to be holding the gun on somebody WITHOUT a shell already in the chamber. IRL, that's the FIRST thing you would do, before even seeing the person you're going to point the gun at!
But of course, the movie and TV people love to use the "clack-clack" as a way to ratchet up the tension level, so they uniformly ignore that bit of common sense, secure in the knowledge that nobody in the audience has ever actually used any sort of firearm.
Just between us, how silly would it look if the guy with the gun had used common sense and actually had a shell already in the chamber when he stepped out to confront the other guy? He'd go "clack-clack" and the perfectly good, unfired shell would be ejected from the gun and go twirling through the air, and land embarrassingly and noisily on the floor between them, making him look like he didn't know the business end of the gun from the butt end.