qapi: Avoid output visitor crashing if it encounters a NULL value

A NULL value is not added to visitor's stack, but there
is no check for that when the visitor tries to return
that value, leading to QEMU crash.

Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>