Fix deprecation notices in PHP >= 8.1

[aanduque]

Context

• Fixes PHP deprecated notice #21455

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where the canonical presenter would throw a deprecation notice when the environments run PHP >= 8.1. Props to aanduque.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• add the following snippet in your theme's functions.php:

add_filter( 'wpseo_canonical', function() { return null; } );
add_filter( 'wpseo_opengraph_title', function() { return null; } );

• make sure you are running PHP >= 8.1
• make sure WP_DEBUG is true
• visit a post in the frontend
• without this patch:
  • you get two trim(): Passing null to parameter #1 ($string) of type string is deprecated warnings
• with this patch:
  • you don´t get the warning above

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes #21455

[enricobattocchi]

Thanks for your contribution @aanduque! This will be released in one of the upcoming versions.