Encode nil datastore keys as PTNull.

service/datastore/serialize/serialize_test.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package serialize
 
 import (
         "bytes"
         "fmt"
         "io"
@@ skipping 46 matching lines @@
                 {
                         "basic",
                         ds.PropertyMap{
                                 "R": {mp(false), mp(2.1), mpNI(3)},
                                 "S": {mp("hello"), mp("world")},
                         },
                 },
                 {
                         "keys",
                         ds.PropertyMap{
-                                "DS":        {mp(mkKey("appy", "ns", "Foo", 7)), mp(mkKey("other", "", "Yot", "wheeep"))},
+                                "DS": {
+                                        mp(mkKey("appy", "ns", "Foo", 7)),
+                                        mp(mkKey("other", "", "Yot", "wheeep")),
+                                        mp((*ds.Key)(nil)),
+                                },
                                 "blobstore": {mp(blobstore.Key("sup")), mp(blobstore.Key("nerds"))},
                         },
                 },
                 {
                         "geo",
                         ds.PropertyMap{
                                 "G": {mp(ds.GeoPoint{Lat: 1, Lng: 2})},
                         },
                 },
                 {
@@ skipping 424 matching lines @@
                                                 "__ancestor__": {
                                                         ToBytes(mp(fakeKey)),
                                                         ToBytes(mp(fakeKey.Parent())),
                                                 },
                                         })
                                 })
                         })
                 })
         })
 }