Fixes notebook markdown failing to render

What does this MR do and why?

Fixes notebook markdown failing to render

In some cases, the source of a cell is just a string, not an array of strings, so it's not necessary to join.

The error can be seen when opening https://gitlab.com/gitlab-org/gitlab-test/-/blob/add-ipython-files/files/ipython/basic.ipynb?ref_type=heads

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

Navigate to http://localhost:3000/gitlab-org/gitlab-test/-/blob/add-ipython-files/files/ipython/basic.ipynb?ref_type=heads, it should display as the fixed version