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

spec/requests/categories_controller_spec.rb

@@ -334,6 +334,7 @@ describe CategoriesController do
 
       describe "success" do
         it "updates attributes correctly" do
+          SiteSetting.tagging_enabled = true
           readonly = CategoryGroup.permission_types[:readonly]
           create_post = CategoryGroup.permission_types[:create_post]
           tag_group = Fabricate(:tag_group)
@@ -415,6 +416,22 @@ describe CategoriesController do
           expect(category.num_auto_bump_daily).to eq(10)
           expect(category.navigate_to_first_post_after_read).to eq(true)
         end
+
+        it "can remove required tag group" do
+          SiteSetting.tagging_enabled = true
+          category.update!(required_tag_group: Fabricate(:tag_group))
+          put "/categories/#{category.id}.json", params: {
+            name: category.name,
+            color: category.color,
+            text_color: category.text_color,
+            allow_global_tags: 'false',
+            min_tags_from_required_group: 1
+          }
+
+          expect(response.status).to eq(200)
+          category.reload
+          expect(category.required_tag_group).to be_nil
+        end
       end
     end
   end