Do not materialize empty varargs[untyped] arrays (#8715)

When an empty nkArgList `varargs[untyped]` is passed around it is now
reused for efficiency sake and to prevent the introduction of a spurious
element: before this commit we'd pass the caller a
nkArgList[nkHiddenStdConv[nkBracket]] node instead of just an empty
nkArgList.

Fixes #8706

tests/macros/t8706.nim

@@ -0,0 +1,21 @@
+discard """
+  output: '''0
+0
+'''
+"""
+
+import macros
+
+macro varargsLen(args:varargs[untyped]): untyped =
+  doAssert args.kind == nnkArglist
+  doAssert args.len == 0
+  result = newLit(args.len)
+
+template bar(a0:varargs[untyped]): untyped =
+  varargsLen(a0)
+
+template foo(x: int, a0:varargs[untyped]): untyped =
+  bar(a0)
+
+echo foo(42)
+echo bar()