Fix remote status fetching for “pure” ActivityPub WEB_ACCOUNT users (#5372)

Remote ActivityPub users that have never been known as OStatus users
(whether or not they support it) will not have a “remote_url” attribute
set. In case they reside on an instance with WEB_DOMAIN ≠ LOCAL_DOMAIN,
the current check did rely on “remote_url” to verify the user's domain.