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

core/fpdfapi/page/fpdf_page_func.cpp

Index: core/fpdfapi/page/fpdf_page_func.cpp
diff --git a/core/fpdfapi/page/fpdf_page_func.cpp b/core/fpdfapi/page/fpdf_page_func.cpp
index a2e31f94bb51fc37fc94ff420e630c721481b493..df658844cdd9ba3f106f7017b2d8ddb15cbfc607 100644
--- a/core/fpdfapi/page/fpdf_page_func.cpp
+++ b/core/fpdfapi/page/fpdf_page_func.cpp
@@ -751,7 +751,8 @@ std::unique_ptr<CPDF_Function> CPDF_Function::Load(CPDF_Object* pFuncObj) {
     pFunc.reset(new CPDF_PSFunc());
 
   if (!pFunc || !pFunc->Init(pFuncObj))
-    return std::unique_ptr<CPDF_Function>();
+    return nullptr;
+
   return pFunc;
 }