=str #15960 Fix broken FlowGraphCompileSpec

* FlowGraphCompileSpec broke when merging in the unzip on top of
  the variance adjustements
* JunctionOutPort[T] must not be defined like JunctionOutPort[+T]
  because then the NextT = Nothing doesn't work
* Do we have a reason to use JunctionOutPort[+T]? I added +T
  only for symmetry reasons with JunctionInPort[-T].
Merge pull request #16008 from akka/wip-15960-variance-take2-patriknw

=str #15960 Fix broken FlowGraphCompileSpec