"JSON path value from HTTP API" Input should allow HTTP method:post with empty body.

## What?

Currently the "JSON path value from HTTP API" input disallows having an empty HTTP body when choosing HTTP method `POST`.

I tried to find a reference where an RFC explicitly declared that empty content is allowed for POST but I was unable to find anything concrete. No RFCs disallow this.

Graylog's own API makes use of POST with an empty body

## Why?

If the data source requires POST with an empty body, this input won't be able to consume that data.

## Your Environment

* Graylog Version: 6.3