FFmpegResultReader throws exception for warnings

[kokorin]

ffmpeg -ss 0 -t 1 -i short.wav -y -f null -c copy NUL
gives the following output:

size=N/A time=00:00:00.00 bitrate=N/A speed=N/A
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)

-c:v copy or removing -t 1 fix output