Problem/Motivation
Currently if you define a container element that is empty, you get empty markup. This may or may not be desirable, but currently you cannot define this behavior.
This was pointed out on #2892304: Introduce footer region to ContentEntityForm
Proposed resolution
Implement #optional property for the Container render element, in the same manner as Details.
Remaining tasks
Do it
User interface changes
None
API changes
New method added: Container:preRenderContainer.
Data model changes
None