Remove empty "attachment" metadata entry in pre-commit notebook cleanup

[christopherbunn]

When opening a Jupyter notebook in VSCode, occasionally it will add an empty "attachment" field in the metadata section as seen here. It is fairly easy to miss out and accidently commit this as part of a review. We should update our Jupyter notebook check to remove this field (if empty, otherwise leave it in if it has data) prior to committing. This code should run in docs/notebook_version_standardizer.py.