qdict: Add qdict_put_null() helper, and put it to use

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20170825105913.4060-2-marcandre.lureau@redhat.com>
[Update to qobject.cocci squashed in, commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>