Empty picture shaders should draw nothing.

src/core/SkPictureShader.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 "SkPictureShader.h"
 
 #include "SkBitmap.h"
@@ skipping 96 matching lines @@
     , fPicture(SkRef(picture))
     , fTile(tile ? *tile : picture->cullRect())
     , fTmx(tmx)
     , fTmy(tmy) {
 }
 
 SkPictureShader::~SkPictureShader() {
     fPicture->unref();
 }
 
-SkPictureShader* SkPictureShader::Create(const SkPicture* picture, TileMode tmx, TileMode tmy,
+SkShader* SkPictureShader::Create(const SkPicture* picture, TileMode tmx, TileMode tmy,
                                          const SkMatrix* localMatrix, const SkRect* tile) {
     if (!picture || picture->cullRect().isEmpty() || (tile && tile->isEmpty())) {
-        return NULL;
+        return SkShader::CreateEmptyShader();
     }
     return SkNEW_ARGS(SkPictureShader, (picture, tmx, tmy, localMatrix, tile));
 }
 
 SkFlattenable* SkPictureShader::CreateProc(SkReadBuffer& buffer) {
     SkMatrix lm;
     buffer.readMatrix(&lm);
     TileMode mx = (TileMode)buffer.read32();
     TileMode my = (TileMode)buffer.read32();
     SkRect tile;
@@ skipping 47 matching lines @@
         if (scaledSize.width() > maxTextureSize || scaledSize.height() > maxTextureSize) {
             SkScalar downScale = maxTextureSize / SkMax32(scaledSize.width(), scaledSize.height());
             scaledSize.set(SkScalarFloorToScalar(SkScalarMul(scaledSize.width(), downScale)),
                            SkScalarFloorToScalar(SkScalarMul(scaledSize.height(), downScale)));
         }
     }
 #endif
 
     SkISize tileSize = scaledSize.toRound();
     if (tileSize.isEmpty()) {
-        return NULL;
+        return SkShader::CreateEmptyShader();
     }
 
     // The actual scale, compensating for rounding & clamping.
     SkSize tileScale = SkSize::Make(SkIntToScalar(tileSize.width()) / fTile.width(),
                                     SkIntToScalar(tileSize.height()) / fTile.height());
 
     SkAutoTUnref<SkShader> tileShader;
     BitmapShaderKey key(fPicture->uniqueID(),
                         fTile,
                         fTmx,
                         fTmy,
                         tileScale,
                         this->getLocalMatrix());
 
     if (!SkResourceCache::Find(key, BitmapShaderRec::Visitor, &tileShader)) {
         SkBitmap bm;
         bm.setInfo(SkImageInfo::MakeN32Premul(tileSize));
         if (!cache_try_alloc_pixels(&bm)) {
-            return NULL;
+            return SkShader::CreateEmptyShader();
         }
         bm.eraseColor(SK_ColorTRANSPARENT);
 
         // Always disable LCD text, since we can't assume our image will be opaque.
         SkCanvas canvas(bm, SkSurfaceProps(0, kUnknown_SkPixelGeometry));
 
         canvas.scale(tileScale.width(), tileScale.height());
         canvas.translate(-fTile.x(), -fTile.y());
         canvas.drawPicture(fPicture);
 
@@ skipping 103 matching lines @@
     return bitmapShader->asFragmentProcessor(context, paint, viewM, NULL, paintColor, fp);
 }
 #else
 bool SkPictureShader::asFragmentProcessor(GrContext*, const SkPaint&, const SkMatrix&,
                                           const SkMatrix*, GrColor*,
                                           GrFragmentProcessor**) const {
     SkDEBUGFAIL("Should not call in GPU-less build");
     return false;
 }
 #endif