A change sometime ago moved to setting some signature fields in the incoming
object to undefined as opposed to deleting them. The trouble is that downstream
code checks against existence, not undefinedness and rejects the message.
Resolves: #9665
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.
