Fix a bug where a URLRequest will hang when empty HTTP/3 response trailers

are received.

The code was checking `received_trailers().empty()` to determine if trailers
had yet been received. However, that doesn't work in this case, obviously.
Instead it can check `trailing_headers_frame_len_ == 0` which is correct.

BUG=332587381

Signed-off-by: Ryan Hamilton <rch@chromium.org>
Change-Id: If2b57fed2c393091e5fe62b8097733afb365224c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5421968


Reviewed-by: Renjie Tang <renjietang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282194}