Check if flow doesn't have sinks after we build it

[dieu]

Hey,

When we work with FlowDef and Execution.fromFn, writes not inserted directly into flowDef anymore, so we need to buildFlow first and then check if the flow doesn't have sinks. Otherwise, we will always have empty flowDef.

[dieu]

@johnynek added the test for check use case, but unfortunately, I have to change the signature of buildFlow from Try[Flow[_]] to Try[Option[Flow[_]]] because we can have totally empty flowDef in case of toIterableExcution when we don't need to do anything only addIter. what do you think?

We can use special exception and keep the signature as is, but I'm not sure that's a good approach.