rgw: return MalformedXML for empty objects list in DeleteObjects

rgw: return MalformedXML for empty objects list in DeleteObjects

When a request contains an empty list of objects, the current
implementation returns a 200 OK. However, this behavior may raise
security concerns, as it could imply the user has access to the
bucket, even though access policies are only evaluated per object.
To mitigate this risk, returning a 400 Bad Request would be a more
secure approach. This ensures that no assumption is made about the
user’s access to the bucket or its objects. Furthermore, this
adjustment aligns with AWS behavior, enhancing compatibility.

Fixes: https://tracker.ceph.com/issues/68799
Signed-off-by: Seena Fallah <seenafallah@gmail.com>