Return NULL when building empty LayerRasterizer.

tests/LayerRasterizerTest.cpp

Index: tests/LayerRasterizerTest.cpp
diff --git a/tests/LayerRasterizerTest.cpp b/tests/LayerRasterizerTest.cpp
index 8e6b64c2076db47b45a5a0efe2e84485f75528d5..640c5813d4bbcd9fa364a8081a5b713ad780931d 100644
--- a/tests/LayerRasterizerTest.cpp
+++ b/tests/LayerRasterizerTest.cpp
@@ -81,6 +81,7 @@ static bool equals(const SkLayerRasterizer_Rec& rec1, const SkLayerRasterizer_Re
 
 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) {
@@ -136,3 +137,8 @@ DEF_TEST(LayerRasterizer_copy, reporter) {
         }
     }
 }
+
+DEF_TEST(LayerRasterizer_detachEmpty, reporter) {
+    SkLayerRasterizer::Builder builder;
+    REPORTER_ASSERT(reporter, NULL == builder.detachRasterizer());
+}