Encode nil datastore keys as PTNull.

service/datastore/properties.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 datastore
 
 import (
         "bytes"
         "encoding/base64"
         "errors"
@@ skipping 306 matching lines @@
                         o = v.Bytes()
                 }
         case reflect.Struct:
                 if t == typeOfTime {
                         tim := v.Interface().(time.Time)
                         if !tim.IsZero() {
                                 o = RoundTime(v.Interface().(time.Time))
                         }
                 }
         }
+
+        switch t {
+        case typeOfKey:
+                if v.IsNil() {
+                        return nil
+                }
+        }
         return o
 }
 
 // Value returns the current value held by this property. It's guaranteed to
 // be a valid value type (i.e. `p.SetValue(p.Value(), true)` will never return
 // an error).
 func (p *Property) Value() interface{} {
         switch p.propType {
         case PTBytes:
                 return p.value.(byteSequence).bytes()
@@ skipping 635 matching lines @@
                 if string(s) == string(t) {
                         return 0, true
                 }
                 if string(s) < string(t) {
                         return -1, true
                 }
                 return 0, true
         }
         return 0, false
 }