Initialize base class avifRWData with {nullptr, 0}

The initializer avifRWData{nullptr, 0} seems more obvious than
avifRWData(), avifRWData{}, or avifRWData({}).

tests/gtest/aviftest_helpers.h

diff --git a/tests/gtest/aviftest_helpers.h b/tests/gtest/aviftest_helpers.h
index a0e58ba..adea4f9 100644
--- a/tests/gtest/aviftest_helpers.h
+++ b/tests/gtest/aviftest_helpers.h
@@ -22,7 +22,7 @@
 
 class AvifRwData : public avifRWData {
  public:
-  AvifRwData() : avifRWData({}) {}
+  AvifRwData() : avifRWData{nullptr, 0} {}
   ~AvifRwData() { avifRWDataFree(this); }
 };