spapr_drc: Expose 'null' in qom-get when there is no fdt

Now that the QMP output visitor supports an explicit null
output, we should utilize it to make it easier to diagnose
the difference between a missing fdt ('null') vs. a
present-but-empty one ('{}').

(Note that this reverts the behavior of commit ab8bf1d, taking
us back to the behavior of commit 6c2f9a1 [which in turn
stemmed from a crash fix in 1d10b44]; but that this time,
the change is intentional and not an accidental side-effect.)

Signed-off-by: Eric Blake <eblake@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <1461879932-9020-17-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
