Fix browser scrollbar showing up for notebooks with only one cell

[jtpio]

Also not sure where we got it from, but it looks like the Notebook now shows a scrollbar by default even when there is a single cell only. This seems to be the case in Notebook 7.1.x too, so not related to this PR in particular:

notebook-default-scrollbar.webm

Originally posted by @jtpio in #7321 (comment)

[jtpio]

This likely comes from https://github.com/jupyterlab/jupyterlab/pull/15533/files#diff-ccf3f53605be6603fd5fd949742f25ea8919c48dcc75322c483ebf9988404c98