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.

script/import_scripts/discuz_x.rb

@@ -887,7 +887,7 @@ class ImportScripts::DiscuzX < ImportScripts::Base
                               LIMIT 1",
         )
 
-      return discuzx_link unless results.size > 0
+      return discuzx_link if results.size.zero?
 
       linked_post_id = results.first["pid"]
       lookup = topic_lookup_from_imported_post_id(linked_post_id)