UDF restore endpoints not to throw error on empty payload

Slack with James Lee:

Hi Marco, just had a QQ about UDF restore. Would it be possible to have the endpoint accept a payload without UDFs. As an example:
If I hit the endpoint on a new cluster with no UDFs, I get given

{"udfMagic":"4D6172636F2072756C6573","udfs":[],"version":"0x01"}

When I then try to restore that payload, I see

{"caller":"admin_accounting_endpoint:1204","code":1040,"icause":"missing function definition or body","key":"service.io.request.bad_value","message":"Error processing UDF restore body"}

This behavior does make sense, but it would be nice if the API could detect that no UDFs were found, then return early (i.e. it doesn't have anything that needs restoring)
I ask this for two separate reasons:
1) It means the backup/restore endpoint provides/accepts the same payload (this is the same behavior from other services from what I recall - I've only tested Analytics at the moment)
2) It means that cbbackupmgr is able to refrain from inspecting the payload and changing behavior conditionally depending on what it contains (ideally, cbbackupmgr should just be a transport mechanism, it doesn't need to be aware of the metadata being backed up)