Use the default question summary if the question note is blank.

question.php

@@ -271,8 +271,11 @@ class qtype_stack_question extends question_graded_automatically {
     }
 
     public function get_question_summary() {
+        if ('' !== $this->questionnoteinstantiated) {
             return $this->questionnoteinstantiated;
         }
+        return parent::get_question_summary();
+    }
 
     public function summarise_response(array $response) {
         $bits = array();