fix wrong error for iterators with no body and pragma macro (#24440)

fixes #16413

`semIterator` checks if the original iterator passed to it has no body,
but it should check the processed node created by `semProcAux`.

tests/pragmas/titeratormacro.nim

@@ -0,0 +1,17 @@
+# issue #16413
+
+import std/macros
+
+macro identity(x: untyped) =
+  result = x.copy()
+  result[6] = quote do:
+    yield 1
+  discard result.toStrLit
+
+iterator demo(): int {.identity.}
+iterator demo2(): int {.identity.} = discard # but this works as expected
+
+var s: seq[int] = @[]
+for e in demo():
+  s.add(e)
+doAssert s == @[1]