Fix deprecation warning by checking for null before stripslashes() in Meta_Description_Presenter

[jilllynndesign]

Context

This PR fixes a deprecation warning introduced in PHP 8.1+ where stripslashes() throws a warning when passed null. The issue occurs in Meta_Description_Presenter when $meta_description is null before being passed to stripslashes(). This can prevent pages using ACF Flexible Content fields from saving properly, showing a "The response is not a valid JSON response" error.

Summary

Changelog:
changelog: bugfix Fixes a bug where saving posts with ACF flexible content fields could fail with a JSON error due to a deprecation warning in PHP 8.1+ (stripslashes(null) in Meta_Description_Presenter).

Relevant technical choices

Added a null check before calling stripslashes():

$meta_description = $this->helpers->string->strip_all_tags(
	$meta_description === null ? null : \stripslashes( $meta_description )
);

Test instructions

1. Install ACF and create a field group using Flexible Content fields.
2. On PHP 8.1 or later, try saving a post using these fields with the current Yoast plugin — confirm that you get the JSON error and see a deprecation warning related to stripslashes()
3. Apply this PR.
4. Save the post again — confirm the error and warning are gone.

Impact check

This PR affects how meta descriptions are handled in Yoast SEO, specifically in Meta_Description_Presenter. Any code that relies on filtering or modifying meta descriptions could be impacted. Regression testing is advised around:

• Meta description outputs
• Open Graph description tags
• Meta description previews in the editor