empty accessCheck() in MakeUniqueEntityFieldTest.php

$query in this case is not an entity query but a Prophecy stub which is being used to satisfy a caller in a test. https://github.com/phpspec/prophecy?tab=readme-ov-file#stubs

What this line is doing is saying that calling accessCheck on the query object will return the same query object; for the purposes of the test we don't care what accessCheck does or what the arguments should be.

thanks @longwave