Resolves #5515

I am not sure though if this could break some streams, because metadata is not treated optionally, so if it's missing, then the validation schema could fail.

@SecondaryH could you please check some more streams (preferrably with a login - which I don't have) and verify these changes? Thanks!
https://github.com/streamlink/streamlink/blob/master/CONTRIBUTING.md#pull-request-feedback

Some streams I've checked:

1. From the tests file:

streamlink --json https://live.nicovideo.jp/watch/co2619719 best | jq .metadata

{
  "id": "lv342666086",
  "author": "novoll(千葉県民)",
  "category": null,
  "title": "【SFC】魔法騎士レイアース 初見プレイ【13枠目】"
}

2. Picked randomly from their site

streamlink --json https://live.nicovideo.jp/watch/lv342666014 best | jq .metadata

{
  "id": "lv342666014",
  "author": "ノンケのねねし",
  "category": null,
  "title": "【スト６】9月16日、17日に仙台で大規模大会があります（ラウンジ）【ノンケ配信記】"
}