suppress deprecated warning on php 8.2.0 because $post_id is null on 404 pages

[oguilleux]

This pull request fixes issue regarding deprecated warnings for php 8 because $post_id is passed null in str_replace and preg_replace.

It will apply the following changes :

• make sure $post_id is always defined in Helpers/original_option_id method
• make sure $post_id is always defined in Main/get_default_reference method

[mikkosiikaniemi]

We are seeing similar PHP deprecated warnings appearing after upgrading from PHP 7.4 to PHP 8.1. Suggesting merging this pull request.

Deprecated: str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in /wp-content/plugins/acf-options-for-polylang/classes/helpers.php on line 38

Deprecated: preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in /wp-content/plugins/acf-options-for-polylang/classes/main.php on line 58