+str #15946 Support flows without junctions in FlowGraph

* run simple flow with source/sink without any junctions in FlowGraph
* partial flow graph with undefined source/sink and flows, without junctions
* replace undefined source/sink with flow with source/sink
Merge pull request #16010 from akka/wip-15946-simple-flow-graphs-patriknw

+str #15946 Support flows without junctions in FlowGraph