fix(cms): 修复导航模型为空时导致的空指针异常

- CmsNavigationServiceImpl中增加model非空判断，防止model为null时报错
- 新增cms_model表中“案例(case)”模型数据，避免选择此模型时报错
- 保障后台栏目导航选择case模型时的保存操作正常执行
- 优化saveAsync方法，提升对未知模型的兼容性和稳定性

src/main/java/com/gxwebsoft/cms/service/impl/CmsNavigationServiceImpl.java

@@ -102,7 +102,7 @@ public class CmsNavigationServiceImpl extends ServiceImpl<CmsNavigationMapper, C
       navigation.setComponent(MessageFormat.format("/pages/{0}/{1}", navigation.getModel(), "[id].vue"));
 
       // 1.2自定义文件后缀
-      if(!navigation.getModel().equals("index") && model.getSuffix() != null){
+      if(!navigation.getModel().equals("index") && model != null && model.getSuffix() != null){
         navigation.setPath(navigation.getPath() + model.getSuffix());
       }