为User problems_status 添加默认值，并精简了相关代码

submission/views.py

@@ -47,10 +47,7 @@ class SubmissionAPIView(APIView):
             except Exception:
                 return error_response(u"提交判题任务失败")
             # 修改用户解题状态
-            if request.user.problems_status:
-                problems_status = json.loads(request.user.problems_status)
-            else:
-                problems_status = {}
+            problems_status = json.loads(request.user.problems_status)
             problems_status[str(data["problem_id"])] = 2
             request.user.problems_status = json.dumps(problems_status)
             request.user.save()