fix bug

app/views/exercises/_user_exercise_info.json.jbuilder

@@ -54,7 +54,7 @@ json.exercise_questions do
       this_ques_status = all_question_status.detect {|f| f[:q_id] == q.id}
       json.answer_status this_ques_status[:stand_status]
     end
-    json.user_score user_ques_answers[:user_score].to_s
+    json.user_score user_ques_answers[:user_score].present? ? user_ques_answers[:user_score].to_s : nil
     json.partial! "exercise_questions/exercise_questions",
                   question: q,
                   ex_answerer: ex_answerer,