Api returns null media_attachments for replies

[connyduck]

I know you can't add attachments to replies on Pixelfed, but the Mastodon api specifies media_attachments as non-null. Pixelfed should just return an empty array instead of null.

This causes crahes in client apps, e.g. tuskyapp/Tusky#1781

https://docs.joinmastodon.org/entities/status/

Example json, received from the https://pixelfed.social/api/v1/notifications endpoint. Note the missing media_attachments on the status object.

JSON

 {
    "id": "445880",
    "type": "mention",
    "created_at": "2020-06-06T00:02:50.000000Z",
    "account": {
      "id": "172618787449868288",
      "username": "ibon",
      "acct": "ibon",
      "display_name": "ibon",
      "locked": false,
      "created_at": "2020-05-31T08:05:26.000000Z",
      "followers_count": 3,
      "following_count": 5,
      "statuses_count": 5,
      "note": "oc memes cha cha real smooth",
      "url": "https:\/\/pixelfed.social\/ibon",
      "avatar": "https:\/\/pixelfed.social\/storage\/avatars\/017\/261\/878\/744\/986\/828\/8\/SUFX3lFK91aQMF34GPc3_avatar.jpeg?v=2",
      "avatar_static": "https:\/\/pixelfed.social\/storage\/avatars\/017\/261\/878\/744\/986\/828\/8\/SUFX3lFK91aQMF34GPc3_avatar.jpeg?v=2",
      "header": "",
      "header_static": "",
      "emojis": [],
      "moved": null,
      "fields": null,
      "bot": false,
      "software": "pixelfed",
      "is_admin": false
    },
    "status": {
      "id": "174671664158740480",
      "created_at": "2020-06-06T00:02:49.000000Z",
      "in_reply_to_id": "174250273865338880",
      "in_reply_to_account_id": 1138,
      "sensitive": false,
      "spoiler_text": "",
      "visibility": "public",
      "language": "en",
      "uri": "https:\/\/pixelfed.social\/p\/ibon\/174671664158740480",
      "url": "https:\/\/pixelfed.social\/p\/ibon\/174671664158740480",
      "replies_count": 0,
      "reblogs_count": 0,
      "favourites_count": 0,
      "reblogged": false,
      "favourited": false,
      "muted": false,
      "bookmarked": false,
      "pinned": false,
      "content": "So calming ❤️",
      "reblog": null,
      "application": {
        "name": "web",
        "website": null
      },
      "mentions": [],
      "tags": [],
      "emojis": [],
      "card": null,
      "poll": null,
      "account": {
        "id": "172618787449868288",
        "username": "ibon",
        "acct": "ibon",
        "display_name": "ibon",
        "locked": false,
        "created_at": "2020-05-31T08:05:26.000000Z",
        "followers_count": 3,
        "following_count": 5,
        "statuses_count": 5,
        "note": "oc memes cha cha real smooth",
        "url": "https:\/\/pixelfed.social\/ibon",
        "avatar": "https:\/\/pixelfed.social\/storage\/avatars\/017\/261\/878\/744\/986\/828\/8\/SUFX3lFK91aQMF34GPc3_avatar.jpeg?v=2",
        "avatar_static": "https:\/\/pixelfed.social\/storage\/avatars\/017\/261\/878\/744\/986\/828\/8\/SUFX3lFK91aQMF34GPc3_avatar.jpeg?v=2",
        "header": "",
        "header_static": "",
        "emojis": [],
        "moved": null,
        "fields": null,
        "bot": false,
        "software": "pixelfed",
        "is_admin": false
      }
    }
  }