check_username_for_spam's documentation incorrectly claims that avatar_url and display_name are never None

[squahtx]

Description

The Synapse documentation claims that the check_username_for_spam spam checker callback receives a userprofile that is a Dict[str, str]. In practice, there can be Nones in the userprofile dict, eg.

{
    avatar_url: None, 
    display_name: None, 
    user_id: '@[redacted]:matrix.org'
}

See https://sentry.matrix.org/sentry/synapse-matrixorg/issues/243697/ for examples of userprofiles with missing avatars and display names.

docs/modules/spam_checker_callbacks.md and CHECK_USERNAME_FOR_SPAM_CALLBACK could do with an update.

[DMRobertson]

We didn't detect this because the function which fetches the data from the db is annotated to return JsonDict.

I would really like a systematic approach to catch this kind of thing.