rolling_update: default ceph json output to empty dict

rolling_update: default ceph json output to empty dict

So we can avoid the following failure:

The conditional check 'hostvars[mon_host]['ansible_hostname'] in (ceph_health_raw.stdout | from_json)["quorum_names"] or hostvars[mon_host]['ansible_fqdn'] in (ceph_health_raw.stdout | from_json)["quorum_names"]
' failed. The error was: No JSON object could be decoded

We just need to set a default, the next iteration will have a more
complete json since the command won't fail.

Signed-off-by: Sébastien Han <seb@redhat.com>