Commit 9d55380b "qapi: Remove null from schema language" (v4.2.0)
neglected to update two error messages.  Do that now.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210224101442.1837475-1-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>