修改 questionNumberEndIsBracket，控制为空情况

src/views/exercise_questions/create/components/create.vue

@@ -159,7 +159,7 @@ export default {
     },
     /**
      * 选择练习
-     * @param {Number} i 练习索引
+     * @param {number} i 练习索引
      */
     selectExerciseItem(i) {
       if (this.indexList.length <= 0) return;

src/views/exercise_questions/preview/components/PreviewMixin.js

@@ -59,6 +59,7 @@ const PreviewMixin = {
      * @returns {string} 题目序号
      */
     questionNumberEndIsBracket(question_number) {
+      if (question_number.length <= 0) return '';
       return `${question_number}${/[()（）]/.test(question_number) ? '' : /[一二三四五六七八九十百]/.test(question_number) ? '、' : '.'}`;
     },
     /**