Describe the bug
httpv2 append extra useless information in get_small_file api

To Reproduce
Steps to reproduce the behavior:

1. set in fe.conf enable_http_server_v2 = true
2. create file in doris
3. call get_small_file api
4. See error

this is because httpv2 will append {"msg":"success","code":0,"data":null,"count":0} to response