=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].

Refs #15960