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

Review-Url: https://codereview.chromium.org/2453163002
diff --git a/fpdfsdk/fpdfdoc_unittest.cpp b/fpdfsdk/fpdfdoc_unittest.cpp
index fc85404..48be723 100644
--- a/fpdfsdk/fpdfdoc_unittest.cpp
+++ b/fpdfsdk/fpdfdoc_unittest.cpp
@@ -25,7 +25,7 @@
 
 class CPDF_TestDocument : public CPDF_Document {
  public:
-  CPDF_TestDocument() : CPDF_Document(std::unique_ptr<CPDF_Parser>()) {}
+  CPDF_TestDocument() : CPDF_Document(nullptr) {}
 
   void SetRoot(CPDF_Dictionary* root) { m_pRootDict = root; }
   CPDF_IndirectObjectHolder* GetHolder() { return this; }