Flood system needs to handle NULL identifiers

Problem/Motivation

In the flood system

  public function register($name, $window = 3600, $identifier = NULL) {
    if (!isset($identifier)) {
      $identifier = $this->requestStack->getCurrentRequest()->getClientIp();
    }
    // We can't use REQUEST_TIME here, because that would not guarantee
    // uniqueness.
    $time = microtime(TRUE);
    $this->events[$name][$identifier][] = ['expire' => $time + $window, 'time' => $time];
  }

But the implementation for Request::getClientIp() does this

    public function getClientIp(): ?string
    {
        return $this->getClientIps()[0];
    }

Which means $identifier can still be null which is a problem. Especially in PHP 8.5 where this triggers a deprecation ... and this is why because it is helping us find a bug.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet