There is no such filter built-in, but it would be very easy for an extension to create a "reply count" filter/gambit that can take a number as parameter, directly a custom gambit name that searchers for zero replies.
I'm not sure what you mean by Flarum sort? Gamification adds a custom sorting option for number of votes, but it won't fully work until the next Flarum release because a PR needs to be merged in Flarum core to enable new sort options during initial page load.
