I'd be more concerned about those huge floating white blobs than a clicking noise. When those fall down, they're liable to hurt someone. Or maybe it needs to be interpreted as part of the medium? What sound effect label would you attach to attempting to fire a safed pistol anyway?

I suppose he could have gone with empty instead of safety on, but that would arguably be a less funny/absurd sequence of events and I don't think it's unreasonable to say that absolute realism was likely a lower priority than the actual point of the comic.