fix(ui测试): 修复UI场景批量执行时报错问题

--bug=1023277 --user=郭雨琦
https://www.tapd.cn/55049933/bugtrace/bugs/view/1155049933001023277

framework/sdk-parent/sdk/src/main/java/io/metersphere/metadata/service/FileMetadataService.java

@@ -252,7 +252,8 @@ public class FileMetadataService {
             request.setUpdateTime(fileMetadata.getUpdateTime());
             bytes = fileManagerService.downloadFile(request);
         }
-        return bytes;
+
+        return Objects.requireNonNullElseGet(bytes, () -> new byte[0]);
     }
 
     public ResponseEntity<byte[]> getFile(String fileId) {