If we want to communicate that a post has replies, but not show the number of them, it's stretching the limits of what an icon can symbolize.
Currently a single arrow just means "click reply to this post", and a double-arrow means that the post itself is a reply to another. Now you want me to make it an empty arrow if it has no replies, and filled if it does, but I think because it's a button it will make people believe replies are disabled when it's empty. Numbers seem more intuitive there.
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