null-stream.nu as stdlib-candidate (#649)

This is a way to redirect to a null stream with the same syntax
regardless of platform.

stdlib-candidate/null-stream.nu

@@ -0,0 +1,10 @@
+# Examples
+#     redirect to the null stream
+#     > echo foo out> (null-stream)
+def null-stream []: nothing -> string {
+    if $nu.os-info.name  == "windows" {
+        "NUL:"
+    } else {
+        "/dev/null"
+    }
+}