Add empty audio track when posting video to telegram

[tonsky]

Videos with no audio are displayed as gif, inline, in small size with no controls.

[zhkvia]

I use this approach:

#!/bin/sh

in=$1
out=$2

ffmpeg -f lavfi -i anullsrc=channel_layout=stereo:sample_rate=44100 -i $1 -shortest -c:v copy -c:a aac $2