Add custom page when there is no license

app/controllers/admin/licenses_controller.rb

 class Admin::LicensesController < Admin::ApplicationController
   before_action :license, only: [:show, :download, :destroy]
-  before_action :require_license, only: [:show, :download, :destroy]
+  before_action :require_license, only: [:download, :destroy]
 
   respond_to :html
 
   def show
-    @previous_licenses = License.previous
+    if @license.blank?
+      render :missing
+    else
+      @previous_licenses = License.previous
+    end
   end
 
   def download

app/views/admin/licenses/missing.html.haml

+- page_title "License"
+
+%h3.page-title
+  Your License
+  = link_to 'Buy License', "#{Gitlab::SUBSCRIPTIONS_URL}/plans", target: '_blank', class: "btn btn-new pull-right"
+  = link_to 'Upload New License', new_admin_license_path, class: "btn pull-right"
+
+%hr
+
+%h4 You do not have a license.
+
+%p You can start a 30 day free trial for all premium features.
+
+= link_to 'Start free trial', new_trial_url, class: "btn btn-new"
+

spec/controllers/admin/licenses_controller_spec.rb

@@ -15,4 +15,26 @@ describe Admin::LicensesController do
       expect(flash[:alert]).to include 'Please enter or upload a license.'
     end
   end
+
+  describe 'GET #show' do
+    context 'with an existent license' do
+      it 'renders the license details' do
+        allow(License).to receive(:current).and_return(create(:license))
+
+        get :show
+
+        expect(response).to render_template(:show)
+      end
+    end
+
+    context 'without a license' do
+      it 'renders missing license page' do
+        allow(License).to receive(:current).and_return(nil)
+
+        get :show
+
+        expect(response).to render_template(:missing)
+      end
+    end
+  end
 end