qapi: add explicit null to string input and output visitors

This may be used for deprecated object properties that are kept for
backwards compatibility.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Tested-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>