no options show for 'Searched fields' if the only field is 'rendered_item'

Problem/Motivation

Steps to reproduce

I made a simple search index, with just the rendered_item field.

Then I added a Search API page and selected that index.

No options shows in the 'Searched fields' form element.

If I go back to the index and add a field, say the node title, and reload the Search API Page's form, then both fields show.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes