Description: currently the http2 codec submits trailers to nghttp2 with this function call:

. In the case that the passed in HeaderMap& is empty, that function call reports ASAN failures: reference binding to null pointer of type 'nghttp2_nv'.

nghttp2 checks for length of the vector before dereferencing, so the code behaves correctly. However, we should clean up trailer submission so that empty trailers don't create ASAN failures.