Test helper: Plugin toggler functionality seems obsolete

[enricobattocchi]

The plugin toggler functionality doesn't seem to work anymore. I'm pretty sure it is obsolete since we made it so that both plugins are active at the same time. It used to be that Premium totally included Free.

For reference, I noted this when testing the PHP 8.1 improvements.
Where Vraja pointed to the last PR changing the functionality.

[amboutwe]

Please inform the customer of conversation # 1285103 when this conversation has been closed.

[amboutwe]

Re: # 1285103

The customer moved wp-content into a separate folder. When enabling this feature, a fatal error appears in the debug.log. Reproduced in a local test environment. Fix requires changing or removing the database value.

PHP Fatal error: Uncaught TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in /www/wp-content/plugins/yoast-test-helper/src/plugin-toggler.php:334