Add uncategorized_description site setting

app/serializers/basic_category_serializer.rb

@@ -17,4 +17,8 @@ class BasicCategorySerializer < ApplicationSerializer
   def include_parent_category_id?
     parent_category_id
   end
+
+  def description
+    object.uncategorized? ? SiteSetting.uncategorized_description : object.description
+  end
 end

config/locales/server.en.yml

@@ -607,6 +607,7 @@ en:
     min_private_message_title_length: "Minimum title length for a private message in characters"
     min_search_term_length: "Minimum search term length in characters"
     allow_uncategorized_topics: "Allow topics to be created without a category."
+    uncategorized_description: "The description of the uncategorized category. Leave blank if you don't want a description."
     allow_duplicate_topic_titles: "Allow topics with identical titles"
     unique_posts_mins: "How many minutes before a user can make a post with the same content again"
     enforce_global_nicknames: "Enforce global username uniqueness (WARNING: only change this during initial setup)"

config/site_settings.yml

@@ -293,6 +293,7 @@ posting:
     client: true
     default: true
     refresh: true
+  uncategorized_description: ''
   allow_duplicate_topic_titles: false
   min_title_similar_length:
     client: true