Fix product visibility evaluation in PHP 8

[felipeelia]

Description of the Change

Currently, InstantResults::apply_product_visibility() calls wc_get_product_visibility_term_ids() and uses $product_visibility_terms['exclude-from-search'] as an array, when it actually is a term id. In PHP 8, that results in a fatal error:

PHP Fatal error: Uncaught Error: Cannot use a scalar value as an array in /var/www/html/wp-content/plugins/elasticpress/includes/classes/Feature/InstantResults/InstantResults.php:479

This PR addresses that, forcing the result to be an array.

Verification Process

• Spin up a local env using PHP 8
• Install WooCommerce and enable "Hide out of stock items from the catalog" (/wp-admin/admin.php?page=wc-settings&tab=products&section=inventory)
• Install the EP Proxy and activate the Instant Results feature
• See the sync failing

Changelog Entry

Fixed the sync page when WC's "hide out of stock items" and Instant Results are both enabled.