Fix a bug of synctex edit command

[naoh16]

This PR would be a solution for the following issues:

• 186

• 200

Their problem are occured when the notebook_dir is a blank, because the generated path is an absolute path from the root: e.g. '/sample.tex'.

NOTE: This is a same solution of the implementation in build_synctex_view_cmd() function.

https://github.com/jupyterlab/jupyterlab-latex/blob/77beeb8b19a08a43f03d02113db5c36eb3874c4c/jupyterlab_latex/synctex.py#L108-L118

[github-actions[bot]]

:point_left: Launch a Binder on branch _naoh16/jupyterlab-latex/fixsynctex