deploy fails when there is a partition without a filesystem

[Marian Gasparovic]

Using Sunbeam deployment with MAAS. During cluster deploy sunbeam fails with

```
                      File "/snap/openstack/479/lib/python3.10/site-packages/sunbeam/provider/maas/commands.py", line 770, in list_machines_cmd
                        machines = list_machines(client)
                      File "/snap/openstack/479/lib/python3.10/site-packages/sunbeam/provider/maas/client.py", line 209, in list_machines
                        machines.append(_convert_raw_machine(machine))
                      File "/snap/openstack/479/lib/python3.10/site-packages/sunbeam/provider/maas/client.py", line 171, in _convert_raw_machine
                        if fs.get("label") == "root":
                    AttributeError: 'NoneType' object has no attribute 'get'
           WARNING An unexpected error has occurred. Please run 'sunbeam inspect' to generate an inspection report. utils.py:290
           ERROR Error: 'NoneType' object has no attribute 'get'
```
There is a partition without a filesystem on each node and this seems to cause the issue.

Side note - recommended `sunbeam inspect` also fails.

[Marian Gasparovic (marosg)]

2023.2/edge

[Guillaume Boutry (gboutry)]

Fix committed, and released to 2024.1/edge: https://github.com/canonical/snap-openstack/pull/227