do not link to root prefix with S3B_ROOT_DIR enabled

[az0]

My configuration is

S3B_ROOT_DIR = 'dl/'

So the tool should not generate the first navigation hyperlink to prefix= which is an empty directory with no link back to dl/

<div id="navigation"><a href="?prefix=">https://www.example.com</a> / <a href="?prefix=dl/">dl</a> / <a href="?prefix=dl//"></a></div>

(This is the breadcrumb navigation above the file listing)

BTW, thanks for the great tool :)