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

core/fpdfdoc/cpdf_annotlist.cpp

Index: core/fpdfdoc/cpdf_annotlist.cpp
diff --git a/core/fpdfdoc/cpdf_annotlist.cpp b/core/fpdfdoc/cpdf_annotlist.cpp
index b626115de19238d3d75dd9763d455dbe7bcf8757..4821f8e6dd6b2b1680d4c853cc98f6868c44a490 100644
--- a/core/fpdfdoc/cpdf_annotlist.cpp
+++ b/core/fpdfdoc/cpdf_annotlist.cpp
@@ -25,13 +25,13 @@ std::unique_ptr<CPDF_Annot> CreatePopupAnnot(CPDF_Annot* pAnnot,
                                              CPDF_Document* pDocument) {
   CPDF_Dictionary* pParentDict = pAnnot->GetAnnotDict();
   if (!pParentDict)
-    return std::unique_ptr<CPDF_Annot>();
+    return nullptr;
 
   // TODO(jaepark): We shouldn't strip BOM for some strings and not for others.
   // See pdfium:593.
   CFX_WideString sContents = pParentDict->GetUnicodeTextFor("Contents");
   if (sContents.IsEmpty())
-    return std::unique_ptr<CPDF_Annot>();
+    return nullptr;
 
   CPDF_Dictionary* pAnnotDict =
       new CPDF_Dictionary(pDocument->GetByteStringPool());