add testcase for #9466 (#17538)

tests/notnil/tnotnil5.nim

@@ -0,0 +1,28 @@
+discard """
+  matrix: "--threads:on"
+"""
+
+{.experimental: "parallel".}
+{.experimental: "notnil".}
+import threadpool
+
+type
+  AO = object
+    x: int
+
+  A = ref AO not nil
+
+proc process(a: A): A =
+  return A(x: a.x+1)
+
+proc processMany(ayys: openArray[A]): seq[A] =
+  var newAs: seq[FlowVar[A]]
+
+  parallel:
+    for a in ayys:
+      newAs.add(spawn process(a))
+  for newAflow in newAs:
+    let newA = ^newAflow
+    if isNil(newA):
+      return @[]
+    result.add(newA)