media: ffmpeg: Add empty AvPacket constructor.
media: ffmpeg: Add empty AvPacket constructor.

The buffer will be allocated by FFmpeg on-demand. This will be used in
encoding scenarios.

BUG=b:239897269
TEST=cargo test --features "video-decoder,ffmpeg" -p devices -p ffmpeg

Change-Id: Ie1e83a6a1129ea666bb3580a669beb7c6cbc8c10
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3920260
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm