fix(ext/node): don't delay http2 client EOF when there's no body
The previous commit buffered the trailing null EOF along with body data
until 'response', but a no-body response (END_STREAM on headers) pushes
only the null. Deferring that null by a tick let a peer that tears the
stream down immediately (e.g. test-http2-compat-socket.js destroys the
socket) race ahead, dropping the client 'end' event.
Only buffer once actual body data is queued; a lone null EOF is already
ordered after 'response' by the nextTick queue, so let it pass through.
