Provide better experience when POSTing an empty body on an endpoint that only has optional body params

[zephraph]

Recently we added /v1/disks/{disk}/finalize to Oxide's API which has a post body of { snapshot_name: Option<Name> }. One might assume that you could simply not specify a POST body when calling the endpoint and it would be handled correctly. That assumption is wrong. Currently the empty string of the POST gets passed to serde which gives us a lovely error of unable to parse JSON body: EOF while parsing a value at line 1 column 0. Now if you know that serde is responsible for parsing the POST body and you know that an empty string is invalid JSON then this error makes complete sense. I do not expect consumers of our API (or other APIs using dropshot) to have that knowledge.

We have a few options here.

1. We could make an update such that dropshot understands that a Body can be optional. This is the ideal implementation imo because it just works™. The best error is no error.
2. We just improve the error message just to inform folks that the body can't be empty. This is likely much easier and a perfectly fine bridge between where we are and where we want to be.