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.

未指定文件、测试或入口点。使用仅包含一个空 contentBlock 的 section 重现该问题，然后跟踪 section/contentBlock 编辑 UI；当空块显示可编辑控件并且可以填充内容时，即完成此项工作。

由索引模型根据 Issue 内容生成。