* remove empty file

If the endpoint is to be implemented later, the file can be added back,
but for now it is confusing to have an empty file.

* enhance(doc): document defaults

Default for `isPublic` is based on the database schema default value.
Defaults for `local` and `withFiles` are based on the behaviour of the endpoint.

* enhance(doc): explain nullable emoji category

* fix: make nullable if default is null

* enhance(doc): explain mute attribute expiresAt

* fix: define required fields

- `notes/create`: the default for `text` has been removed because ajv can not handle
  `default` inside of `anyOf`, see
  https://ajv.js.org/guide/modifying-data.html#assigning-defaults


  and the default value cannot be `null` if text is `nullable: false` in the `anyOf`
  first alternative.
- `notes/create`: The `mediaIds` property has been marked as deprecated because it
  has the same behaviour as using `fileIds`, but the implementation tries to handlè
  `fileIds` first.
- The result schema for `admin/emoji/list` has been altered because the `host`
  property will always be `null` as it is filtered this way in the database query.
  See packages/backend/src/server/api/endpoints/admin/emoji/list.ts line 67.

* enhance(doc): explain nullable hostname

* update changelog

Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>