avoid zero-as-null-pointer warning (#7423)

Co-authored-by: Derek Bailey <derekbailey@google.com>

tests/test.cpp

@@ -625,7 +625,7 @@ void SizePrefixedTest() {
   // Create size prefixed buffer.
   flatbuffers::FlatBufferBuilder fbb;
   FinishSizePrefixedMonsterBuffer(
-      fbb, CreateMonster(fbb, 0, 200, 300, fbb.CreateString("bob")));
+      fbb, CreateMonster(fbb, nullptr, 200, 300, fbb.CreateString("bob")));
 
   // Verify it.
   flatbuffers::Verifier verifier(fbb.GetBufferPointer(), fbb.GetSize());