A sociopath feels no such tension, since they regard you or me the same way we would regard a bicycle: as a tool to be used. Ergo, no expression. (I’ve seen this up close, and been struck by it.)
Now think of the way your face feels when you’re watching a somewhat boring, late night rerun on TV which is gradually lulling you to sleep. You feel zero social stress, so you’re completely relaxed, and are wearing no expression. At that moment, you, too, have the “blank, empty eyes” of a sociopath.
The difference is, that is not the expression you wear for social situations.
Having said all this, I should also point out that sociopaths are usually consummate actors, and can often feign emotions so convincingly that they appear more heartfelt than those of non-sociopaths who are actually feeling those emotions.
But, if you know what to look for, sociopaths will also always give themselves away in various ways, and one of those ways is that they sometimes wear a completely blank, empty expression.