Handle missing attachment key

[jimgoo]

Fixes this error when opening OutputExamples.ipynb.

 File "/Users/jimgoo/miniconda3/envs/jlab-fork/lib/python3.8/site-packages/jupyter_server_ydoc/ydoc.py", line 249, in maybe_save_document
    if model["content"] != self.room.document.source:
  File "/Users/jimgoo/miniconda3/envs/jlab-fork/lib/python3.8/site-packages/jupyter_ydoc/ydoc.py", line 26, in source
    return self.get()
  File "/Users/jimgoo/miniconda3/envs/jlab-fork/lib/python3.8/site-packages/jupyter_ydoc/ydoc.py", line 140, in get
    if cell["cell_type"] in ["raw", "markdown"] and not cell["attachments"]:
KeyError: 'attachments'

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[davidbrochart]

Thanks @jimgoo !

[welcome]

Congrats on your first merged pull request in this project! 🎉

Thank you for contributing, we are very proud of you! ❤️

[davidbrochart]

FYI it looks like your git user information is not properly linked to your GitHub account, since you don't appear in contributors.

[jimgoo]

Thanks for the heads up @davidbrochart, I've updated my git account info locally.