tmpfs storage config definitions fail

[David Tulloh]

Running an install (Jammy) with an install definition that includes

```
autoinstall:
...
  storage:
    config:
...
    - id: tmpfs
      type: mount
      spec: "none"
      path: "/tmp"
      fstype: "tmpfs"
      options: "mode=1777,nosuid,nodev"
```

Leads to the following error

```
File "/snap/subiquity/5495/lib/python3.10/site-packages/curtin/commands/block_meta.py", line 1251, in mount_data
             raise ValueError(
         ValueError: mount entry without 'device' missing: ['fstype']. ({'path': '/tmp', 'options': 'mode=1777,nosuid,nodev', 'spec': 'none', 'id': 'tmpfs', 'type': 'mount'})
```

I traced this back to the curtin invocation in /var/log/installer/curtin-install/subiquity-partitioning.conf which contains

```
  - path: /tmp
    options: mode=1777,nosuid,nodev
    spec: none
    id: tmpfs
    type: mount
```

Critically, the fstype entry has been removed, leading me to believe that the error is in the subiquity code.

This previously worked with earlier Jammy images.

Currently running subiquity 24.02.1 via the image at https://cdimage.ubuntu.com/ubuntu-server/jammy/daily-live/current/

A potentially related bug report from 2023-10-28 https://bugs.launchpad.net/curtin/+bug/2041725

I believe this was broken by this commit https://github.com/canonical/subiquity/commit/0cc4fdf34893d7420cd3bf5d227b273791dbcdfb

The commit removed the fstype parameter, the new code assumes a device, which does not work in the case of tmpfs when the device is None.