Use nbformat when creating an empty cell

[davidbrochart]

Problem

When we set the content of a YNotebook from some source, we create an empty cell if there was no cell originally.

Proposed Solution

We should probably use nbformat for that.

[krassowski]

What is the status on this? I see that the linked code is no longer here. Asking because I am trying to narrow down some trust issues which appear to originate from ydoc and friends.

Otherwise, I am not sure if spawning up nbformat up is a good idea performance-wise

[davidbrochart]

This issue is not fixed, we are still creating an empty cell here.