HTTP/2: stop sending empty DATA frame when sending trailers.
HTTP/2: stop sending empty DATA frame when sending trailers.

Previously, empty DATA frame was sent to the client, followed by trailers,
when trailers were generated and there was no payload to send.

The issue was introduced in 3db1fc3.

Change-Id: Ibba0a0b021b12c689dc5c72277389e28c6ce9991
Signed-off-by: Piotr Sikora <ba6f3a1a6d7e1ecae91b31e93969cc392fbd0fdd@google.com>
Reviewed-on: https://nginx-review.googlesource.com/1280
Reviewed-by: Martin Maly <e8c0ee8c9f9b3872b26a60336f45a625d658b541@google.com>

firebase/nginx,firebase/nginx,firebase/nginx,firebase/nginx