DEV: Enable unless cops

We discussed the use of `unless` internally and decided to enforce
available rules from rubocop to restrict its most problematic uses.

plugins/discourse-narrative-bot/lib/discourse_narrative_bot/actions.rb

@@ -49,7 +49,7 @@ module DiscourseNarrativeBot
           SiteSetting.rate_limit_create_post
         end
 
-      return unless duration > 0
+      return if duration <= 0
 
       data = DiscourseNarrativeBot::Store.get(user.id.to_s)
       return unless data