openstack overcloud ceph deploy --network-data could better handle an invalid network_data_v2.yaml file

The error [1] was seen after running the following command with [2] as network_data_v2.yaml:

    openstack overcloud ceph deploy \
              --ceph-spec ceph_spec.yaml \
              --skip-user-create \
              -y -o deployed_ceph.yaml \
              --network-data network_data_v2.yaml \
              --container-namespace quay.io/ceph \
              --container-image daemon \
              --container-tag v6.0.6-stable-6.0-pacific-centos-8-x86_64 \
              --stack overcloud

[1]
~~~
Exception occured while running the command
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/tripleoclient/command.py", line 34, in run
    super(Command, self).run(parsed_args)
  File "/usr/lib/python3.6/site-packages/osc_lib/command/command.py", line 39, in run
    return super(Command, self).run(parsed_args)
  File "/usr/lib/python3.6/site-packages/cliff/command.py", line 186, in run
    return_code = self.take_action(parsed_args) or 0
  File "/usr/local/lib/python3.6/site-packages/tripleoclient/v2/overcloud_ceph.py", line 323, in take_action
    parsed_args.cluster_network_name)
  File "/usr/local/lib/python3.6/site-packages/tripleoclient/utils.py", line 3273, in get_ceph_networks
    subnet = _get_subnet(net, ip_subnet)
  File "/usr/local/lib/python3.6/site-packages/tripleoclient/utils.py", line 3251, in _get_subnet
    return ','.join(ip_subnets)
TypeError: sequence item 0: expected str instance, NoneType found
sequence item 0: expected str instance, NoneType found
~~~

[2]
~~~
- name: Storage
  name_lower: storage
  subnets:
    storage_subnet:
      enable_dhcp: true
      ipv6_address_mode: dhcpv6-stateful
      ipv6_allocation_pools:
      - end: fd00:fd00:fd00:3000:aaaa:aaaa:ffff:fffe
        start: fd00:fd00:fd00:3000::10
      ipv6_ra_mode: dhcpv6-stateful
      ipv6_subnet: fd00:fd00:fd00:3000::/64
      physical_network: storage
      vlan: 30
  vip: true
- name: StorageMgmt
  name_lower: storage_mgmt
  subnets:
    storage_mgmt_subnet:
      enable_dhcp: true
      ipv6_address_mode: dhcpv6-stateful
      ipv6_allocation_pools:
      - end: fd00:fd00:fd00:4000:aaaa:aaaa:ffff:fffe
        start: fd00:fd00:fd00:4000::10
      ipv6_ra_mode: dhcpv6-stateful
      ipv6_subnet: fd00:fd00:fd00:4000::/64
      physical_network: storage_mgmt
      vlan: 40
  vip: true
- name: InternalApi
  name_lower: internal_api
  subnets:
    internal_api_subnet:
      enable_dhcp: true
      ipv6_address_mode: dhcpv6-stateful
      ipv6_allocation_pools:
      - end: fd00:fd00:fd00:2000:aaaa:aaaa:ffff:fffe
        start: fd00:fd00:fd00:2000::10
      ipv6_ra_mode: dhcpv6-stateful
      ipv6_subnet: fd00:fd00:fd00:2000::/64
      physical_network: internal_api
      vlan: 20
  vip: true
- name: Tenant
  name_lower: tenant
  subnets:
    tenant_subnet:
      allocation_pools:
      - end: 172.17.2.149
        start: 172.17.2.10
      ip_subnet: 172.17.2.0/24
      physical_network: tenant
      vlan: 50
- name: External
  name_lower: external
  vip: true
  subnets:
    external_subnet:
      enable_dhcp: true
      gateway_ipv6: 2620:52:0:13b8::fe
      ipv6_address_mode: dhcpv6-stateful
      ipv6_allocation_pools:
      - end: 2620:52:0:13b8:5054:ff:fe3e:aa
        start: 2620:52:0:13b8:5054:ff:fe3e:1
      ipv6_ra_mode: dhcpv6-stateful
      ipv6_subnet: 2620:52:0:13b8::/64
      physical_network: external
      vlan: 10
- name: Management
  name_lower: management
  subnets:
    management_subnet:
      enable_dhcp: true
      gateway_ipv6: fd00:fd00:fd00:6000::1
      ipv6_address_mode: dhcpv6-stateful
      ipv6_allocation_pools:
      - end: fd00:fd00:fd00:6000:ffff:ffff:ffff:fffe
        start: fd00:fd00:fd00:6000::10
      ipv6_ra_mode: dhcpv6-stateful
      ipv6_subnet: fd00:fd00:fd00:6000::/64
      physical_network: management
      vlan: 60
~~~