### Problem When we set the content of a YNotebook from some source, we create an [empty cell](https://github.com/jupyter-server/jupyter_ydoc/blob/05d3a05e670da14164aff49293c4ddeb207f7ade/jupyter_ydoc/ydoc.py#L164-L171) if there was no cell originally. ### Proposed Solution We should probably use [nbformat](https://github.com/jupyter/nbformat) for that.