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

When opening a Jupyter notebook in VSCode, occasionally it will add an empty "attachment" field in the metadata section as seen [here](https://github.com/alteryx/evalml/pull/4215#discussion_r1248163024). 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`.