Ensure that the _misskey_content attribute will always exist. Because
the API endpoint does not require the existence of the `text` field,
that field may be `undefined`. By using `?? null` it can be ensured
that the value is at least `null`.
Furthermore, the rendered HTML of a note with empty text will also be
the empty string. From git blame it seems that this behaviour was added
because of a Mastodon bug that might have previously existed. Hoever,
this seems to be no longer the case as I can find mastodon posts that
have empty content.
The code could be made a bit more succinct by using the null coercion
operator.
