The settings form never says that no bundle carries a file field

Problem/Motivation

On a site where no entity type carries a file or image field, the What is read section of the settings form is meant to say No entity type on this site carries a file or image field yet. It never does: the check looks at the first key of the section, which is always #type, so it always concludes that something was offered. The site builder sees an empty section with no explanation.

Found in the review before 1.0.0-alpha1 and again in the review of the release; measured there.

Steps to reproduce

1. On a site with no file or image field on any bundle, open Configuration → Content authoring → Content credentials.
2. The What is read section is empty, without the sentence.

Proposed resolution

Check the section's children instead of its first key, so the sentence appears exactly when nothing is offered. Saving the form on such a site keeps working and stores no participating bundles.

Remaining tasks

• Tests, then the fix.

User interface changes

On a site without file fields, the settings form says so.