\Drupal\Core\Template\AttributeString::__toString() can cause deprecations in PHP 8.1

@daffie #3 fixes it for sure. That's what is in #3220021: [meta] Ensure compatibility of Drupal 9 with PHP 8.1 (as it evolves) - I did #2 to see where and why we have the issue.

@daffie yes we know that already. The patch in #3 is exactly what the patch on #3220021: [meta] Ensure compatibility of Drupal 9 with PHP 8.1 (as it evolves) does to fix this on PHP 8.1.

What I'm trying to explore here is what is the best fix. The first step to that it to understand why $this->value is not a string sometimes. Here's why...

Tests are failing in #2 not because of NULLs but because of integers... for example:

$variables['attributes']['data-striping'] = 1;

in template_preprocess_table() is one cause. Another is checkbox values that are often integers.

I've considered overriding the \Drupal\Core\Template\AttributeValueBase constructor with something like:

  /**
   * {@inheritdoc}
   */
  public function __construct($name, $value) {
    if (!is_string($value)) {
      @trigger_error('\Drupal\Core\Template\AttributeString should only process strings.', E_USER_DEPRECATED);
      $value = (string) $value;
    }
    parent::__construct($name, $value);
  }

This would eventually allow us to add the string typehint to $value but this would have a behaviour change side-effect of changing the type when the raw value is returned by \Drupal\Core\Template\AttributeValueBase::value() so I think we should go for the solution in #3 and cross this bridge when we think more about primitive typehints.