[REST Connector] Make behaviour on empty payload configurable

[Licenser]

trafficstars

In the REST source/onramp, when we encounter a payload that either from preprocessors or codec does not create an event we currently return a 400 Response, this is probably good in most cases but it might be worth being able to tune this behavior.

Feedback with use-cases where this would break is welcome!

[Licenser]

Might be good to do this with the rest connector port

[mfelsche]

Sending a 204 might be a good default, but being able to customize the behaviour would be more desirable. Maybe a config like empty_status_code does the trick, for starters. We unfortunately have no event to execute some tremor-script upon.