Fix undefined error when checkpoints is empty

[jdavid]

      const checkpoints = await context?.listCheckpoints();       <== if checkpoints is an empty array
      if (!checkpoints) {                                         <== this will be false
        return;
      }
      const checkpoint = checkpoints[checkpoints.length - 1];     <== then checkpoint will be undefined
      node.textContent = trans.__(
        'Last Checkpoint: %1',
        Time.formatHuman(new Date(checkpoint.last_modified))      <== here will trigger an undefined error
      );

We found the problem while integrating jupyterlite with a FastAPI project.

[github-actions]

👈 Launch a Binder on branch jdavid/notebook/fix-js-undefined-error

[jtpio]

Thanks!

[jtpio]

@meeseeksdev please backport to 4.3.x

[lumberbot-app]

Something went wrong ... Please have a look at my logs.

It seems that the branch you are trying to backport to does not exist.

[jtpio]

@meeseeksdev please backport to 7.3.x

[jtpio]

We found the problem while integrating jupyterlite with a FastAPI project.

Do you need this change in JupyterLite soon? It may take a little while to propagate there as this would first need to be available in a Notebook release.

Normally this issue should not have too much of an impact, except for some errors in the dev tools console?

[jdavid]

In our case the error breaks the notebook, but I don't need the fix quickly, we already patch the file.
Thanks for merging!