docs/sphinx: Fix TOC scroll hack for the home page

When on the documentation home page, there won't be any ".current"
element since no entry from the TOC was selected yet. That results in a
javascript error. Fix that by only trying to set the scrollTop if we
have matches for current entries.

Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240123162157.61819-2-gustavo.sousa@intel.com