Add check for user value in CollaboratorsPanel

[martinRenou]

Follow up https://github.com/jupyterlab/jupyter-collaboration/pull/367.

The server awareness has no user by default, to avoid seeing it as a collaborator if not required.

[github-actions[bot]]

:point_left: Launch a Binder on branch _martinRenou/jupyter-collaboration/update_collaboratortyping

[brichet]

@martinRenou I added a description.