Notebook base_url configuration always keeps "/" even when configured with empty string

Hi,

I am trying to make the html and other served files by the notebook relative to the entire path. However, the **base_url** configuration always results on the notebook serving files with "/" as the first character.
The html code below is the result of using *jupyter notebook --NotebookApp.base_url=""*
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The issue names no files or tests; first reproduce the behavior with `jupyter notebook --NotebookApp.base_url=""` and inspect how the NotebookApp builds links for served HTML and static files. Done means an empty base_url produces relative links without a leading slash and still supports the described reverse-proxy use case.

Written by the indexing model from the issue text.