Core: Fix error thrown if docs.defaultName is unset

[stilt0n]

Closes #23860

What I did

Fixed a regression that was introduced in #23182 where an invariant was used to enforce defaultName being defined. When defaultName is undefined, it should default to 'Docs' rather than triggering an invariant. See MIGRATION.md here.

How to test

Run a sandbox with:

docs: {
  autodocs: true,
  defaultName: undefined,
}

In the main.ts config. Should not get any invariant messages in the logs.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "build", "documentation", "maintenance", "dependencies", "other"]

[JReinhold]

Thanks for this! 💪

defaultName will be set by addon-docs, so this would only fail if a user doesn't have addon-docs installed and sets autodocs: true, which is a weird case.

However it is valid, and we should just ignore it instead of throwing an error.