Fix `tracking_label` usage

This ensures that `tracking_label`
can be called without a subject.

Additionally, ensures that code using this method
properly calls experimentation framework.

lib/gitlab/experimentation/controller_concern.rb

@@ -140,7 +140,7 @@ module Gitlab
         cookies[:force_experiment].to_s.split(',').any? { |experiment| experiment.strip == experiment_key.to_s }
       end
 
-      def tracking_label(subject)
+      def tracking_label(subject = nil)
         return experimentation_subject_id if subject.blank?
 
         if subject.respond_to?(:to_global_id)

spec/helpers/invite_members_helper_spec.rb

@@ -7,6 +7,10 @@ RSpec.describe InviteMembersHelper do
   let_it_be(:developer) { create(:user, developer_projects: [project]) }
   let(:owner) { project.owner }
 
+  before do
+    helper.extend(Gitlab::Experimentation::ControllerConcern)
+  end
+
   context 'with project' do
     before do
       assign(:project, project)
@@ -202,7 +206,6 @@ RSpec.describe InviteMembersHelper do
 
       before do
         allow(helper).to receive(:experiment_tracking_category_and_group) { '_track_property_' }
-        allow(helper).to receive(:tracking_label)
         allow(helper).to receive(:current_user) { owner }
       end