Handle event streaming when entity does not exist

Instead of raising an error, terminate the
worker execution instead and do not retry.

ee/app/workers/audit_events/audit_event_streaming_worker.rb

@@ -15,6 +15,9 @@ module AuditEvents
 
     def perform(audit_event_id)
       audit_event = AuditEvent.find(audit_event_id)
+
+      return if audit_event.entity.nil?
+
       group = group_entity(audit_event)
 
       return if group.nil? # Do nothing if the event can't be resolved to a single group.

ee/spec/workers/audit_events/audit_event_streaming_worker_spec.rb

@@ -97,5 +97,19 @@ RSpec.describe AuditEvents::AuditEventStreamingWorker do
         worker.perform(event.id)
       end
     end
+
+    context 'when the entity is a NullEntity' do
+      let_it_be(:event) { create(:audit_event, :project_event) }
+
+      before do
+        event.entity_id = non_existing_record_id
+      end
+
+      it 'makes no HTTP calls' do
+        expect(Gitlab::HTTP).not_to receive(:post)
+
+        worker.perform(event.id)
+      end
+    end
   end
 end