WebUI Settings: Navigate away from Site Details with empty site

To prevent users interacting with the Site Details page without having
a site set (which can crash Chrome if the user attempts to reset
permissions) the page is configured to navigate up a level if no site
is provided.

The user can only reach this page without a site by manually typing it
into the address bar.

Bug: 1241396
Change-Id: If48b105e60ee13c08ee894ccab58819954066a25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3114456


Reviewed-by: Sean Harrison <harrisonsean@chromium.org>
Commit-Queue: Theodore Olsauskas-Warren <sauski@google.com>
Cr-Commit-Position: refs/heads/main@{#915102}