Layout Paragraphs Builder (Experimental) not working on new/empty content using Layout Builder

Problem/Motivation

When a node that uses Layout Builder in its display settings has no layout paragraphs content, the Layout Paragraphs Builder (Experimental) does not display. This is especially apparent when using the Mercury Editor since it uses the front theme. Content with existing layout paragraphs content is not a problem.

Steps to reproduce

1. A content type that uses Layout Builder for its display settings
2. Set Layout Paragraphs Builder (Experimental) as the field display
3. Create a new page with Mercury Editor OR
   create a new page, don't populate the Layout Paragraphs field, then view with the front theme.
4. The space where Layout Paragraphs Builder (Experimental) normally shows is empty.
5. Removing Layout Builder from the content type's display settings restores Layout Paragraphs Builder (Experimental) functionality for empty content.

This was not an issue with previous versions. I haven't done a back versions to determine whether this is caused by module or core changes (using 10.3.1).

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes