early-boot-config: handle empty user data

[tjkirch]

We started with k8s variants that required user data to be able to connect to a cluster. Later variants don't necessarily require user data - for example, the ECS variant can use the default cluster. The -dev variants may not need any settings in some use cases.

Today, empty user data is considered an error, for example with an aws-k8s variant you can see early-boot-config fail like this, followed by a number of services failing due to a dependency on early-boot-config, or from missing settings. (Since settings-committer can't run, there are no default settings available, either.)

[    8.869121] early-boot-config[2935]: Provider error: TOML data did not contain 'settings' section