PHP 8.4 deprecates implicitly nullable parameters, i.e. typed parameters with a `null` default value, which are not explicitly declared as nullable.
This commit the one instance of this in the `WP_HTML_Processor` class.
Fixed by adding the nullability operator to the type, which is supported since PHP 7.1, so we can use it now the minimum supported PHP version is PHP 7.2.
As this deprecation is thrown at compile time, it can be seen at the top of the test output when running on PHP 8.4 (which will be gone once this change has been committed). It is not possible to write a test to cover this.
Ref: https://wiki.php.net/rfc/deprecate-implicitly-nullable-types
Follow-up to [58867], [58769], [58304], [58192].
Props jrf.
See #62061.
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