fix(examWords): 处理生成单词为空的异常情况

- 增加判断如果生成的单词对象为空或单词列表为空，则抛出运行时异常
- 防止后续处理中因单词缺失导致的错误
- 提升系统稳定性与异常提示的准确性

enlish-service/src/main/java/com/yinlihupo/enlish/service/controller/ExamWordsController.java

@@ -50,6 +50,9 @@ public class ExamWordsController {
         }
         try {
             ExamWordsDO examWordsDO = examWordsService.generateExamWords(gradeId, level, studentIds);
+            if (examWordsDO == null || examWordsDO.getWordIds().isEmpty()) {
+                throw new RuntimeException("没有单词");
+            }
             List<VocabularyBankDO> vocabularyBankDOS = vocabularyService.findVocabularyBankDOListById(examWordsDO.getWordIds());
             List<Word> assessmentWords = vocabularyBankDOS.stream().map(vocabularyBankDO -> Word.builder()
                     .id(vocabularyBankDO.getId())