This is popping up on GrowthExperiments patches:
```11:15:07 There was 1 error:
11:15:07
11:15:07 1) PageImages\Tests\PageImagesTest::testGivenNonExistingPageOnBeforePageDisplayDoesNotAddMeta
11:15:07 === Logs generated by test case
11:15:07 [objectcache] [debug] MainWANObjectCache using store {class} {"class":"EmptyBagOStuff"}
11:15:07 [localisation] [debug] LocalisationCache: using store LCStoreNull []
11:15:07 ===
11:15:07 Error: Call to a member function response() on null
11:15:07
11:15:07 /workspace/src/includes/OutputPage.php:341
11:15:07 /workspace/src/extensions/PageImages/tests/phpunit/PageImagesTest.php:51
11:15:07 /workspace/src/tests/phpunit/MediaWikiIntegrationTestCase.php:434
11:15:07 /workspace/src/maintenance/doMaintenance.php:99
```
Not related to any specific change, as shown by https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GrowthExperiments/+/546655.
No changes in PageImages recently, and core and other extension patches do not seem affected (maybe GrowthExperiments specifically pulls in PageImages tests in CI and most other repos don't?).