ffmpeg_encoder: unable to encode video with no audio

[flavioribeiro]

with http://flv.io/video.mp4, for example:

{
  "destination": "ftp://lg:pass@flavioribeiro.com/",
  "id": "dcuPgRvku5CKigLp",
  "preset": {
    "audio": {
      "bitrate": "64000",
      "codec": "aac"
    },
    "container": "m3u8",
    "description": "HLS 1080 for tests",
    "name": "my_m3u8_files",
    "rateControl": "vbr",
    "video": {
      "bitrate": "1000000",
      "codec": "h264",
      "gopMode": "fixed",
      "gopSize": "90",
      "height": "240",
      "interlaceMode": "progressive",
      "profile": "main",
      "profileLevel": "3.1",
      "width": "426"
    }
  },
  "progress": "unable to get the best audio stream inside the input context",
  "source": "http://flv.io/video.mp4",
  "status": "error"
}