[remoting] Don't send zero-sized frames to the WebRTC output sink.

WebrtcVideoStream::OnCaptureResult() sometimes receives non-null frames
which have zero size. If these are sent to the output sink, they
cause a DCHECK inside WebRTC's video-stream-encoder. This CL updates
the null check to also reject empty frames.

Bug: 1339384
Change-Id: I10f75004f6e2d8e321b176dd90722e556307aa02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3723716
Commit-Queue: Joe Downing <joedow@chromium.org>
Auto-Submit: Lambros Lambrou <lambroslambrou@chromium.org>
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1017765}