[FIX] Fix browser title not displaying correctly after database update

During Tiki installation with an existing database, the browser title field on the General Settings screen incorrectly displayed "My Tiki" (the default value) instead of the existing title from the database. This issue occurred because the preference loading mechanism wouldn't update preferences that were already set, even with empty values(browser title has an empty value by default). The fix enhances the update_preferences() function to properly update empty preferences with non-empty values from the database, ensuring the browser title is correctly displayed during installation. This prevents users from inadvertently overwriting their existing browser title with the default value, which could negatively impact SEO.

Item: 8780