FIX: unable to remove required tag group from a category

app/controllers/categories_controller.rb

@@ -281,6 +281,7 @@ class CategoriesController < ApplicationController
       if SiteSetting.tagging_enabled
         params[:allowed_tags] ||= []
         params[:allowed_tag_groups] ||= []
+        params[:required_tag_group_name] ||= ''
       end
 
       result = params.permit(

app/models/category.rb

@@ -559,7 +559,7 @@ class Category < ActiveRecord::Base
   end
 
   def required_tag_group_name=(group_name)
-    self.required_tag_group = group_name ? TagGroup.where(name: group_name).first : nil
+    self.required_tag_group = group_name.blank? ? nil : TagGroup.where(name: group_name).first
   end
 
   def downcase_email