graphviz svg links incorrect if referencing same file

[chrisjsewell]

If you have a link like href="#abc", then the current behaviour of fix_svg_relative_paths doesn't work; you need to handle the case here where rel_uri is empty: https://github.com/sphinx-doc/sphinx/blob/116a430caca98580c8877cf7b0ba270eae9604bf/sphinx/ext/graphviz.py#L248