3a1f7761 added a required identifier parameter
to Verifier::VerifyBuffer but did not update the templated call site in
BufferRef. This causes errors like the following when trying to instantiate the
call site by calling BufferRef::Verify():

    include/flatbuffers/flatbuffers.h: In instantiation of ‘bool flatbuffers::BufferRef<T>::Verify() [with T = MyType]’:
    MyApp.cpp:16:3:   required from here
    include/flatbuffers/flatbuffers.h:1421:37: error: no matching function for call to ‘flatbuffers::Verifier::VerifyBuffer()’
         return verifier.VerifyBuffer<T>();
                                         ^
    include/flatbuffers/flatbuffers.h:1421:37: note: candidate is:
    include/flatbuffers/flatbuffers.h:1352:29: note: template<class T> bool flatbuffers::Verifier::VerifyBuffer(const char*)
       template<typename T> bool VerifyBuffer(const char *identifier) {
                                 ^
    include/flatbuffers/flatbuffers.h:1352:29: note:   template argument deduction/substitution failed:
    include/flatbuffers/flatbuffers.h:1421:37: note:   candidate expects 1 argument, 0 provided
         return verifier.VerifyBuffer<T>();
                                         ^

This commit fixes the BufferRef call site by adding a nullptr argument.