qa: fix issue with fn unable to fetch port and ip

qa: fix issue with fn unable to fetch port and ip

_get_port_ip_info() fails to fetch port and ip due to empty 'backend' key:

2023-02-24T20:49:09.084 DEBUG:teuthology.orchestra.run.smithi042:> sudo adjust-ulimits ceph-coverage /home/ubuntu/cephtest/archive/coverage timeout 120 ceph --cluster ceph nfs cluster info test
2023-02-24T20:49:09.471 INFO:teuthology.orchestra.run.smithi042.stdout:{
2023-02-24T20:49:09.472 INFO:teuthology.orchestra.run.smithi042.stdout:  "test": {
2023-02-24T20:49:09.472 INFO:teuthology.orchestra.run.smithi042.stdout:    "backend": [],
2023-02-24T20:49:09.472 INFO:teuthology.orchestra.run.smithi042.stdout:    "virtual_ip": null
2023-02-24T20:49:09.472 INFO:teuthology.orchestra.run.smithi042.stdout:  }
2023-02-24T20:49:09.472 INFO:teuthology.orchestra.run.smithi042.stdout:}

it then raises:

2023-02-24T20:49:10.323 INFO:tasks.cephfs_test_runner:    info_output = json.loads(self._nfs_cmd('cluster', 'info', self.cluster_id))['test']['backend'][0]
2023-02-24T20:49:10.323 INFO:tasks.cephfs_test_runner:IndexError: list index out of range

Signed-off-by: Dhairya Parmar <dparmar@redhat.com>