根据书号查询教材管理信息，解决空指针报错问题

src/main/java/com/xjrsoft/module/textbook/service/impl/TextbookServiceImpl.java

@@ -451,6 +451,9 @@ public class TextbookServiceImpl extends MPJBaseServiceImpl<TextbookMapper, Text
     @Override
     public TextbookVo getInfoByissn(String issn) {
         TextbookVo infoByissn = textbookTextbookMapper.getInfoByissn(issn);
+        if(infoByissn == null){
+            return null;
+        }
         List<TextbookClassRelation> classRelationList = textbookTextbookClassRelationMapper.selectList(
                 new QueryWrapper<TextbookClassRelation>().lambda().eq(TextbookClassRelation::getTextbookId, infoByissn.getId())
         );