Fix string type hints to Container getters for PHP 8.5

Problem/Motivation

Split from 📌 Deal with NULL as array key/offset Needs work

Steps to reproduce

5) /builds/issue/drupal-3523596/core/lib/Drupal/Component/DependencyInjection/Container.php:150
Using null as an array offset is deprecated, use an empty string instead
Triggered by:
* Drupal\Tests\Component\DependencyInjection\ContainerTest::testGetForNonExistentNULLService
  /builds/issue/drupal-3523596/core/tests/Drupal/Tests/Component/DependencyInjection/ContainerTest.php:369
* Drupal\Tests\Component\DependencyInjection\PhpArrayContainerTest::testGetForNonExistentNULLService
  /builds/issue/drupal-3523596/core/tests/Drupal/Tests/Component/DependencyInjection/ContainerTest.php:369

Proposed resolution

add string type hint as interface ContainerInterface extends PsrContainerInterface declares at

Remaining tasks

part, review,commit

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet