contentBlock can't be edited if it's the only thing in a <section>

If a section contains a contentBlock but nothing else, there's no way to edit the contentBlock to seed it with actual content - the empty contentBlock editing box doesn't appear.

If the section contains text, the dashed yellow lines appear, and can be clicked on to edit. Once the contentBlock is non-empty, it shows up fine.

No files, tests, or entry points are named. Reproduce the issue with a section containing only an empty contentBlock, then trace the section/contentBlock editing UI; it is done when the empty block shows an editable control and can be seeded with content.

索引モデルが issue の本文から書いたものです。