Take advantage of implicit std::unique_ptr<>(nulltpr_t) ctor.

core/fpdfapi/render/fpdf_render_pattern.cpp

Index: core/fpdfapi/render/fpdf_render_pattern.cpp
diff --git a/core/fpdfapi/render/fpdf_render_pattern.cpp b/core/fpdfapi/render/fpdf_render_pattern.cpp
index f55a0e8b97e0dcf0f0fb232d3b3c2ee86c9c39db..5238550bf5b953e796726e776566285d3af02dab 100644
--- a/core/fpdfapi/render/fpdf_render_pattern.cpp
+++ b/core/fpdfapi/render/fpdf_render_pattern.cpp
@@ -819,7 +819,7 @@ std::unique_ptr<CFX_DIBitmap> DrawPatternBitmap(
   std::unique_ptr<CFX_DIBitmap> pBitmap(new CFX_DIBitmap);
   if (!pBitmap->Create(width, height,
                        pPattern->colored() ? FXDIB_Argb : FXDIB_8bppMask)) {
-    return std::unique_ptr<CFX_DIBitmap>();
+    return nullptr;
   }
   CFX_FxgeDevice bitmap_device;
   bitmap_device.Attach(pBitmap.get(), false, nullptr, false);