Add missing return type to AsciiSlugger::getLocale()

[Staormin]

Got the following issue this morning in symfony 5.4-RC1 with php 8.1:

!!  
!!  Fatal error: Declaration of Symfony\Component\String\Slugger\AsciiSlugger::getLocale() must be compatible with Symfony\Contracts\Translation\LocaleAwareInterface::getLocale(): string in /opt/project/vendor/symfony/string/Slugger/AsciiSlugger.php on line 93
!!  Symfony\Component\ErrorHandler\Error\FatalError {#6153
!!    #message: "Compile Error: Declaration of Symfony\Component\String\Slugger\AsciiSlugger::getLocale() must be compatible with Symfony\Contracts\Translation\LocaleAwareInterface::getLocale(): string"
!!    #code: 0
!!    #file: "./vendor/symfony/string/Slugger/AsciiSlugger.php"
!!    #line: 93
!!    -error: array:4 [
!!      "type" => 64
!!      "message" => "Declaration of Symfony\Component\String\Slugger\AsciiSlugger::getLocale() must be compatible with Symfony\Contracts\Translation\LocaleAwareInterface::getLocale(): string"
!!      "file" => "/opt/project/vendor/symfony/string/Slugger/AsciiSlugger.php"
!!      "line" => 93
!!    ]
!!  }
!!  

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony repository. It is used by Composer to allow developers to depend on specific Symfony components.

If you want to contribute, you should instead open a pull request on the main repository:

https://github.com/symfony/symfony

Thank you for your contribution!

PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply.

[fabrau]

getLocale cannot return a nullable value ?