处理修改学生报错的问题。

TEAMModelOS.SDK/Models/Service/StudentService.cs

@@ -1737,6 +1737,10 @@ namespace TEAMModelOS.SDK
                                 if (classId != null && classId.Length == 0) no = string.Empty;
 
                                 //classId => 沒給欄位(null) 有給欄位("") 但更新一定得給教室?
+                                if (classId == null)
+                                {
+                                    classId = "";
+                                }
                                 if (classStuds.ContainsKey(classId))
                                 {
                                     classStuds[classId].Add((id.GetString(), salt, pw, name, year, null, gender, null, null, classId, no, periodId));