There are several ways to "output to null" in FFmpeg.

there is the null muxer, like

ffmpeg -i INPUT -f null out.null # file out.null will not be created

There is the null video sink:

https://ffmpeg.org/ffmpeg-filters.html#nullsink (not clear if you can actually use this from the command line)

or just output to /dev/null (or the reserved filename NUL in windows) for your file to just disappear.

ffmpeg -i INPUT -f rawvideo /dev/null # avoids doing any conversion before writing time (may still demuxes and decodes the input though)

All useful in testing/analyzing when you don't want the "writing out" part to get in your way. See also the "-benchmark" parameter.

There may be a way to use the "-map" parameter as well, unclear.