spapr_drc: Change value of property "fdt" from null back to {}

prop_get_fdt() misuses the visitor API: when fdt is null, it doesn't
visit anything.  object_property_get_qobject() happily
object_property_get_qobject().  Amazingly, the latter survives the
misuse.  Turns out we've papered over it long before prop_get_fdt()
existed, in commit 1d10b445.

However, commit 6c2f9a15 changed how we paper over it, and as a side
effect changed qom-get's value from {} to null.  Change it right back
by fixing the visitor misuse.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>