Template part focus mode doesn't handle empty template parts very well

[jameskoster]

I think this is most likely a result of the removal of the appender in #37213.

It is especially problematic when creating a template part from scratch via list view. The resultant editor view makes it impossible to interact with the canvas. It all feels a bit confusing:

empty.mp4

As suggested in #36827, I think it would make sense for the appender (or an empty paragraph) to appear if the template part is empty.

[Mamaduka]

Probably duplicate of #37524.

[jameskoster]

Right you are!