runtime-v2: fix parallel loop execution when no out variable defined

[brig]

flows:
  default:
    - call: EmptyFlow
      out: outVar
      loop:
        items:
          - "one"
          - "two"
        mode: parallel

    - log: "outVar: ${outVar}"  # prints [(this Collection), (this Collection)]

  EmptyFlow:
    - return