When indexable creation is disabled, there's redundant db queries on page load, in the frontend of posts

[leonidasmi]

After #20335 is merged, the fixing of the disabling of indexable creation is going to have a side-effect:

• Upon page load on a post, we're going to have a query that's going to be redundant and always do nothing, because of the fact that we don't have an indexable for that post (this happened only once before, because we were creating an indexable upon page load):

DELETE
FROM `wp_yoast_indexable_hierarchy`
WHERE `indexable_id` = NULL

• We can probably return early before going into the clear_ancestors() function, to avoid that.