[Symfony 6] Drupal\Core\Routing\Router::matchCollection(): Return value must be of type array, null returned

Problem/Motivation

In Drupal, Drupal\Core\Routing\Router::matchCollection() is declared as:

   * @return array|null
   *   An array of parameters. NULL when there is no match.
   */
  protected function matchCollection($pathinfo, RouteCollection $routes) {

In Symfony 6, the signature changes to

    protected function matchCollection(string $pathinfo, RouteCollection $routes): array

ie. we can no longer return NULL when there is no match. The upstream docblock gives a clue as to a possible fix:

     * @throws ResourceNotFoundException If the resource could not be found

Steps to reproduce

Proposed resolution

Convert the NULL return to an exception.

Remaining tasks

See what side effects this change has.
Figure out if this is only affects core or could affect contrib/custom code.
If needed, figure out how to warn downstream users of this change.

User interface changes

API changes

Router::matchCollection() will throw an exception instead of returning NULL when there is no match.

Data model changes

Release notes snippet