Fix PHPStan findings at UserBasedTimezoneDetectorTest::testUserTimezoneDetection()

[phansys]

Subject

PHPStan detects that other values than null are not returned and suggests to update the return type accordingly, but the null return type is supported since PHP 8.2 and we are currently supporting PHP 8.0:

... never returns null so it can be removed from the return type.

I am targeting this branch, because this change respects BC.

See: https://github.com/sonata-project/SonataIntlBundle/actions/runs/7240749104/job/19724081508?pr=602#step:5:11.

[VincentLanglet]

but the null return type is supported since PHP 8.2 and we are currently supporting PHP 8.0:

This can be solved with phpdoc

/**
 * @return null
 */

[phansys]

This can be solved with phpdoc

I was afraid about the covariance, but you're right. The legacy interface has no type declarations.

It's updated. Thank you for the hint!