fix: 自动组卷选题难度增加无选项

src/modules/paper/components/AutomaticPaper.vue

@@ -145,7 +145,8 @@ export default {
     const questionDifficultyMap = [
       { value: '1', label: '易' },
       { value: '2', label: '中' },
-      { value: '3', label: '难' }
+      { value: '3', label: '难' },
+      { value: '0', label: '无' }
     ]
     const defaultItem = {
       question_type: '',