fix bugs

stmms-web/src/main/java/cn/com/qmth/stmms/admin/exam/MarkGroupController.java

@@ -517,7 +517,7 @@ public class MarkGroupController extends BaseExamController {
             mainNumbers.add(question.getMainNumber());
         }
         // 一个分组内只能有一个选做题大题
-        if (mainNumbers.size() > 1) {
+        if (selectives.size() > 0 && mainNumbers.size() > 1) {
             return false;
         }
         if (questionIds.length == selectives.size()) {