Fix nullable type declaration

[todeveni]

Summary

Fixes deprecation warnings on PHP 8.4.

Since Visualizer requires PHP 7.4 and nullable types are a PHP 7.1 feature, this shouldn't break any backwards compatibility.

Deprecated: Visualizer_Module_Chart::_getChartArray(): Implicitly marking parameter $chart as nullable is deprecated, the explicit nullable type must be used instead in /app/wp-content/plugins/visualizer/classes/Visualizer/Module/Chart.php on line 382

Will affect visual aspect of the product

NO

Test instructions

• Load current plugin using a site with PHP 8.4
• Watch deprecation errors/warnings on log

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)