10 files declare their use of NullBackend in Drupal 9

CachedStorageTest.php in core/tests/Drupal/Tests/Core/Config/CachedStorageTest.php

ContextDefinitionIsSatisfiedTest.php in core/tests/Drupal/Tests/Core/Plugin/Context/ContextDefinitionIsSatisfiedTest.php

ContextHandlerTest.php in core/tests/Drupal/Tests/Core/Plugin/ContextHandlerTest.php

Contains \Drupal\Tests\Core\Plugin\ContextHandlerTest.

EntityContextDefinitionIsSatisfiedTest.php in core/tests/Drupal/Tests/Core/Plugin/Context/EntityContextDefinitionIsSatisfiedTest.php

NullBackendTest.php in core/tests/Drupal/Tests/Core/Cache/NullBackendTest.php

ProviderRepositoryTest.php in core/modules/media/tests/src/Kernel/ProviderRepositoryTest.php

RecursiveContextualValidatorTest.php in core/tests/Drupal/Tests/Core/TypedData/RecursiveContextualValidatorTest.php

Contains \Drupal\Tests\Core\TypedData\RecursiveContextualValidatorTest.

ResourceFetcherTest.php in core/modules/media/tests/src/Unit/ResourceFetcherTest.php

ThemeExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php

UpdateBackend.php in core/lib/Drupal/Core/Update/UpdateBackend.php