$ ffmpeg -f lavfi -i nullsrc -c copy -f null /dev/null "" 2>&1
ffmpeg version 2.8.7 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.2.1 (GCC) 20070831 patched [FreeBSD]
  configuration: --prefix=/usr/local --mandir=/usr/local/man --datadir=/usr/local/share/ffmpeg --pkgconfigdir=/usr/local/libdata/pkgconfig --enable-shared --enable-gpl --enable-postproc --enable-avfilter --enable-avresample --enable-pthreads --disable-libstagefright-h264 --disable-libutvideo --disable-libsoxr --cc=cc --extra-cflags='-msse -I/usr/local/include/vorbis -I/usr/local/include' --extra-ldflags='-L/usr/local/lib ' --extra-libs=-lpthread --enable-memalign-hack --disable-libaacplus --disable-indev=alsa --disable-outdev=alsa --disable-libopencore-amrnb --disable-libopencore-amrwb --disable-libass --disable-libbs2b --disable-libcaca --disable-libcdio --disable-libcelt --disable-libdc1394 --disable-debug --disable-htmlpages --disable-libfaac --enable-libfdk-aac --disable-ffserver --disable-libflite --enable-fontconfig --enable-libfreetype --enable-frei0r --disable-libfribidi --enable-libgme --disable-libgsm --enable-iconv --disable-libilbc --disable-indev=jack --disable-ladspa --enable-libmp3lame --disable-libbluray --enable-mmx --disable-libmodplug --disable-openal --disable-indev=openal --disable-opencl --enable-libopencv --disable-opengl --disable-libopenh264 --disable-libopenjpeg --enable-libopus --disable-libpulse --disable-indev=pulse --disable-outdev=pulse --disable-libquvi --enable-runtime-cpudetect --enable-librtmp --enable-libschroedinger --disable-ffplay --disable-outdev=sdl --disable-libsmbclient --disable-libsnappy --enable-libspeex --enable-sse --disable-libssh --enable-libtheora --disable-libtwolame --disable-libv4l2 --disable-indev=v4l2 --disable-outdev=v4l2 --disable-vaapi --disable-vdpau --disable-libvidstab --enable-libvorbis --disable-libvo-aacenc --disable-libvo-amrwbenc --enable-libvpx --disable-libwavpack --enable-libwebp --disable-x11grab --enable-libx264 --enable-libx265 --disable-libxcb --disable-libxvid --disable-outdev=xv --disable-libzmq --disable-libzvbi --enable-gnutls --disable-openssl --disable-version3 --enable-nonfree
  libavutil      54. 31.100 / 54. 31.100
  libavcodec     56. 60.100 / 56. 60.100
  libavformat    56. 40.101 / 56. 40.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 40.101 /  5. 40.101
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  2.101 /  1.  2.101
  libpostproc    53.  3.100 / 53.  3.100
Input #0, lavfi, from 'nullsrc':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 320x240 [SAR 1:1 DAR 4:3], 25 tbr, 25 tbn, 25 tbc
[NULL @ 0x80a852620] Unable to find a suitable output format for ''
: Invalid argument