Don't send CI usage email notifications when quota is unlimited

ee/app/services/ci_minutes_usage_notify_service.rb

@@ -3,6 +3,7 @@
 class CiMinutesUsageNotifyService < BaseService
   def execute
     return unless ::Gitlab.com?
+    return unless namespace.shared_runners_minutes_limit_enabled?
 
     notify_on_total_usage
     notify_on_partial_usage

ee/changelogs/unreleased/rd-do-not-send-ci-quota-notifications-when-quota-is-unlimited.yml

+---
+title: Don't send CI usage email notifications when quota is unlimited
+merge_request: 14810
+author:
+type: fixed

ee/spec/services/ci_minutes_usage_notify_service_spec.rb

@@ -189,6 +189,18 @@ describe CiMinutesUsageNotifyService do
         it_behaves_like 'no notification is sent'
       end
 
+      context 'when quota is unlimited' do
+        # Gitlab.com? => true is required in order to excercise the notification
+        let(:gitlab_dot_com) { true }
+        let(:ci_minutes_used) { 1500 }
+
+        before do
+          namespace.update_attribute(:shared_runners_minutes_limit, 0)
+        end
+
+        it_behaves_like 'no notification is sent'
+      end
+
       it_behaves_like 'notification for custom level is sent', 1500, 30
 
       context 'when other Pipeline has finished but second level of alert has not been reached' do