🐛 write_files: nil is invalid cloud-config

[joshfrench]

What type of PR is this?
/kind bug

What this PR does / why we need it:
When EKSConfigTemplate.spec.template.spec.files is empty, the resulting cloud config is invalid:

#cloud-config
write_files:
runcmd:
  - /etc/eks/bootstrap.sh cluster-name

cloud-init status --long
status: error
extended_status: error - done
boot_status_code: enabled-by-generator
last_update: Thu, 01 Jan 1970 00:25:50 +0000
detail: Failed due to systemd unit failure
errors:
        - ('write_files', TypeError("'NoneType' object is not iterable"))

Special notes for your reviewer:

This may only be an issue on newer versions of cloud-init (or maybe the underlying system Python?) We've been using CAPA for years and only saw this problem after updating our AMIs to Ubuntu 24.04:

cloud-init --version
/usr/bin/cloud-init 25.2-0ubuntu1~24.04.1

Checklist:

• squashed commits
• includes documentation
• includes emoji in title
• adds unit tests
• adds or updates e2e tests

Release note:

Fixes invalid cloud-config when EKS config has no files.

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