Mysterious fix for #8550 (#8561)

Replacing the `for` body with a nkEmpty node is not the right thing to
do.

tests/closure/t8550.nim

@@ -0,0 +1,12 @@
+discard """
+  output: "@[\"42\"]"
+"""
+
+proc chk_fail(): seq[string] =
+  iterator x(): int {.closure.} = yield 42
+  proc f(cl: iterator(): int {.closure.}): seq[string] =
+    result = @[]
+    for i in cl(): result.add($i)
+  result = f(x)
+
+echo(chk_fail())