Do not download an audio file when not exist

[nikukyugamer]

If an audio file doesn't exist in the target file, master.json's audio property's value is null.

For example: https://wonderpla.net/

{
  "clip_id": "ddd0bdb4-a498-4b6e-bb02-3900fbefe90d",
  "base_url": "../",
  "video": [...],
  "audio": null
}

So json.audio.sort occurs an Uncaught TypeError: Cannot read properties of null error.

I fixed it.

[Tusko]

Thank you @nikukyugamer