Make NotifyService more robust

Be resilient if `alerts` are missing in the payload.

ee/app/services/projects/prometheus/alerts/notify_service.rb

@@ -63,7 +63,7 @@ module Projects
         end
 
         def gitlab_alert_id
-          alerts.first.dig('labels', 'gitlab_alert_id')
+          alerts&.first&.dig('labels', 'gitlab_alert_id')
         end
 
         def compare_token(expected, actual)

ee/spec/services/projects/prometheus/alerts/notify_service_spec.rb

@@ -158,6 +158,12 @@ describe Projects::Prometheus::Alerts::NotifyService do
 
       it_behaves_like 'no notifications'
     end
+
+    context 'with missing alerts' do
+      let(:payload) { { 'version' => '4' } }
+
+      it_behaves_like 'no notifications'
+    end
   end
 
   private