search: do not panic / use Zoekt code path if indexed search is disabled

Created by: slimsag

This PR fixes an issue where disabling indexed search via the site config would result in a nil pointer panic reported in #25269 (closed)

This PR also fixes an issue where the default value of search.index.enabled was incorrectly computed as false on some Docker instances (older ones), while newer ones would always start with "search.index.enabled": true,.

This regression was released in 3.32.0, but having indexed search disabled is rare, this would generally only affect single-container Docker instances, and anyone can easily workaround the issue by adding "search.index.enabled": true, to their site configuration.

Fixes #25269 (closed)

Signed-off-by: Stephen Gutekanst [email protected]