Avoid a compiler compatibility issue with CPDF_DefaultAppearance

Some older Clang compilers get confused by StringTemplate usage in
CPDF_DefaultAppearance's default constructor. Avoid this issue by
removing the default constructor and changing the only caller to use the
1-param ctor instead.

Change-Id: If2b43fdcf94ff5d6f666c83f451f8a9e8fa67339
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/117610
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Thomas Sepez <tsepez@google.com>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/core/fpdfdoc/cpdf_defaultappearance.cpp b/core/fpdfdoc/cpdf_defaultappearance.cpp
index a0abdad..1f78b32 100644
--- a/core/fpdfdoc/cpdf_defaultappearance.cpp
+++ b/core/fpdfdoc/cpdf_defaultappearance.cpp
@@ -54,8 +54,6 @@
 
 }  // namespace
 
-CPDF_DefaultAppearance::CPDF_DefaultAppearance() = default;
-
 CPDF_DefaultAppearance::CPDF_DefaultAppearance(const ByteString& csDA)
     : m_csDA(csDA) {}
 
diff --git a/core/fpdfdoc/cpdf_defaultappearance.h b/core/fpdfdoc/cpdf_defaultappearance.h
index 2ec725c..78180ad 100644
--- a/core/fpdfdoc/cpdf_defaultappearance.h
+++ b/core/fpdfdoc/cpdf_defaultappearance.h
@@ -16,7 +16,6 @@
 
 class CPDF_DefaultAppearance {
  public:
-  CPDF_DefaultAppearance();
   explicit CPDF_DefaultAppearance(const ByteString& csDA);
   CPDF_DefaultAppearance(const CPDF_DefaultAppearance& cDA);
   ~CPDF_DefaultAppearance();
diff --git a/core/fpdfdoc/cpdf_interactiveform.cpp b/core/fpdfdoc/cpdf_interactiveform.cpp
index c9e04c5..fe7c07a 100644
--- a/core/fpdfdoc/cpdf_interactiveform.cpp
+++ b/core/fpdfdoc/cpdf_interactiveform.cpp
@@ -723,9 +723,8 @@
 }
 
 CPDF_DefaultAppearance CPDF_InteractiveForm::GetDefaultAppearance() const {
-  if (!m_pFormDict)
-    return CPDF_DefaultAppearance();
-  return CPDF_DefaultAppearance(m_pFormDict->GetByteStringFor("DA"));
+  return CPDF_DefaultAppearance(
+      m_pFormDict ? m_pFormDict->GetByteStringFor("DA") : "");
 }
 
 int CPDF_InteractiveForm::GetFormAlignment() const {