If a user has moderate_comment capability and access to the Comment Panel, the Empty Spam button appears, but it doesn't actually empty the spam when clicked.
In general, I don't understand the weirdness in the WordPress core about moderating comments. If a user has moderate_comment capability, they SHOULD have access to the Comments Panel and they should be able to moderate comments. Neither of those really happens. In order to see the Comments Panel, you have to give the user the totally-unrelated edit_posts capability, and even then, the Empty Spam is tied to yet some other capability that I don't even know.