application_helper.rb

module ApplicationHelper
  def title(*parts)
    unless parts.empty? 
      parts << t(:website)
      content_for(:title) do
        parts.join(' - ')
      end
    end
  end
end

application_helper_test.rb

require 'test_helper'

class ApplicationHelperTest < ActionView::TestCase
  include ApplicationHelper
  test "no title should return nil" do
    assert title == nil
  end
  
  test "title(*parts) should generate title" do
    assert title([1, 2, 3, 'This is a test']) == "1 - 2 - 3 - This is a test - WebsiteName"
  end
end

error

ApplicationHelperTest#test_title(*parts)_should_generate_title:
NoMethodError: undefined method `append' for nil:NilClass
    app/helpers/application_helper.rb:3:in `title'
    test/helpers/application_helper_test.rb:11:in `block in <class:ApplicationHelperTest>'