[Bug 1939015] [NEW] [Ubuntu 20.04.2 Server] Autoinstall max() arg failure with cloud-config

[Alexander Turner]

Public bug reported:

I'm PXE booting and auto installing a 20.04 server without success. The
server is acquiring the `user-data` file from my cloud-init server
however it fails at the some point on each install,
`convert_autoinstall_config: max() arg is an empty sequence`

For reference my ipxe params:

```
kernel vmlinuz root=/dev/ram0 ip=dhcp hostname=test url=http://releases.ubuntu.com/20.04.2/ubuntu-20.04.2-live-server-amd64.iso autoinstall ds=nocloud-net;s=http://server/configs/
initrd initrd
```

and more excitingly, my user-data (simple example from Canonical to
test). meta-data is empty.

```
version: 1
identity:
    hostname: test
    username: tester
    password: an_encrypted_password
```
The error:
https://i.stack.imgur.com/7NBWB.png

I'm unable to login to `installer` and access a shell (Ctrl+C ends the
session) to read the file. SCP won't copy the file, so the crash is
currently a mystery.

** Affects: subiquity (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1939015

Title:
  [Ubuntu 20.04.2 Server] Autoinstall max() arg failure with cloud-
  config

Status in subiquity package in Ubuntu:
  New

Bug description:
  I'm PXE booting and auto installing a 20.04 server without success.
  The server is acquiring the `user-data` file from my cloud-init server
  however it fails at the some point on each install,
  `convert_autoinstall_config: max() arg is an empty sequence`

  For reference my ipxe params:

  ```
  kernel vmlinuz root=/dev/ram0 ip=dhcp hostname=test url=http://releases.ubuntu.com/20.04.2/ubuntu-20.04.2-live-server-amd64.iso autoinstall ds=nocloud-net;s=http://server/configs/
  initrd initrd
  ```

  and more excitingly, my user-data (simple example from Canonical to
  test). meta-data is empty.

  ```
  version: 1
  identity:
      hostname: test
      username: tester
      password: an_encrypted_password
  ```
  The error:
  https://i.stack.imgur.com/7NBWB.png

  I'm unable to login to `installer` and access a shell (Ctrl+C ends the
  session) to read the file. SCP won't copy the file, so the crash is
  currently a mystery.

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