修复评卷任务查询时该科目没有分组的bug

stmms-web/src/main/webapp/WEB-INF/views/modules/exam/libraryList.jsp

@@ -164,7 +164,7 @@ $('#group-select').change(function(){
     $('#marker-select').empty();
     $('#marker-select').append('<option value="0">不限</option>');
     $('#marker-select').val('0').trigger('change');
-    if(subjectCode=='' || groupNumber==''){
+    if(subjectCode=='' || groupNumber==''||groupNumber==null){
         return;
     }
     $.post('${ctx}/admin/exam/marker/query', {subjectCode: subjectCode, groupNumber: groupNumber}, function(result){