If policy file is empty, mc requests the user provides a Content-Length header

[MrCreosote]

Expected behavior

When attempting to create a policy with the mc client, a empty policy document should cause the client to error out with a helpful error message.

Actual behavior

The mc client tells the user to provide a Content-Length header, which is not very helpful to the user, especially a newbie.

Steps to reproduce the behavior

~$ cat badpolicy.json 
~$ ~/minio/mc/RELEASE/2024-11-21T17-21-54Z/mc admin policy create local9000 badpolicy badpolicy.json
mc: <ERROR> Unable to create new policy. You must provide the Content-Length HTTP header.

mc --version

~$ ~/minio/mc/RELEASE/2024-11-21T17-21-54Z/mc admin info local9000
●  localhost:9000
   Uptime: 3 minutes 
   Version: 2024-12-18T13:15:44Z
*snip*
~$ ~/minio/mc/RELEASE/2024-11-21T17-21-54Z/mc -v
mc version RELEASE.2024-11-21T17-21-54Z (commit-id=1681e4497c09d7438a34e846f76dbde972ab7daf)
Runtime: go1.23.4 linux/amd64
Copyright (c) 2015-2024 MinIO, Inc.
License GNU AGPLv3 <https://www.gnu.org/licenses/agpl-3.0.html>

System information

Ubuntu 18 VM running on an Ubuntu 18 laptop. Upgrade plans are in motion

Other notes

I haven't tested with malformed JSON or valid JSON but mismatched with the policy schema - not sure what error messages those might return