qapi: Permit 'boxed' with empty type

We reject empty types with 'boxed': true.  We don't really need that
to work, but making it work is actually simpler than rejecting it, so
do that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190913201349.24332-9-armbru@redhat.com>
11 files changed