Return NULL when building empty LayerRasterizer.

tests/LayerRasterizerTest.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDeque.h"
 #include "SkLayerRasterizer.h"
 #include "SkPaint.h"
@@ skipping 63 matching lines @@
     SkPaint     fPaint;
     SkVector    fOffset;
 };
 
 static bool equals(const SkLayerRasterizer_Rec& rec1, const SkLayerRasterizer_Rec& rec2) {
     return rec1.fPaint == rec2.fPaint && rec1.fOffset == rec2.fOffset;
 }
 
 DEF_TEST(LayerRasterizer_copy, reporter) {
     SkLayerRasterizer::Builder builder;
+    REPORTER_ASSERT(reporter, NULL == builder.snapshotRasterizer());
     SkPaint paint;
     // Create a bunch of paints with different flags.
     for (uint32_t flags = 0x01; flags < SkPaint::kAllFlags; flags <<= 1) {
         paint.setFlags(flags);
         builder.addLayer(paint, static_cast<SkScalar>(flags), static_cast<SkScalar>(flags));
     }
 
     // Create a layer rasterizer with all the existing layers.
     SkAutoTUnref<SkLayerRasterizer> firstCopy(builder.snapshotRasterizer());
 
@@ skipping 35 matching lines @@
         recDetached  = static_cast<const SkLayerRasterizer_Rec*>(iterDetached.next());
 
         REPORTER_ASSERT(reporter, equals(*recOneLarger, *recDetached));
         if (smallerCount == i) {
             REPORTER_ASSERT(reporter, recFirstCopy == NULL);
         } else {
             REPORTER_ASSERT(reporter, equals(*recFirstCopy, *recOneLarger));
         }
     }
 }
+
+DEF_TEST(LayerRasterizer_detachEmpty, reporter) {
+    SkLayerRasterizer::Builder builder;
+    REPORTER_ASSERT(reporter, NULL == builder.detachRasterizer());
+}