Empty picture shaders should draw nothing.

src/core/SkPictureShader.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkPictureShader_DEFINED
 #define SkPictureShader_DEFINED
 
 #include "SkShader.h"
 
 class SkBitmap;
 class SkPicture;
 
 /*
  * An SkPictureShader can be used to draw SkPicture-based patterns.
  *
  * The SkPicture is first rendered into a tile, which is then used to shade the area according
  * to specified tiling rules.
  */
 class SkPictureShader : public SkShader {
 public:
-    static SkPictureShader* Create(const SkPicture*, TileMode, TileMode, const SkMatrix*,
+    static SkShader* Create(const SkPicture*, TileMode, TileMode, const SkMatrix*,
                                    const SkRect*);
     virtual ~SkPictureShader();
 
     size_t contextSize() const override;
 
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkPictureShader)
 
     bool asFragmentProcessor(GrContext*, const SkPaint&, const SkMatrix& viewM, const SkMatrix*,
                              GrColor*, GrFragmentProcessor**) const override;
@@ skipping 32 matching lines @@
         SkShader::Context*      fBitmapShaderContext;
         void*                   fBitmapShaderContextStorage;
 
         typedef SkShader::Context INHERITED;
     };
 
     typedef SkShader INHERITED;
 };
 
 #endif // SkPictureShader_DEFINED