Fixes the mail locale check

[Godmartinz]

The mail locale check was looking for a false when it should of been looking for a true on if ($event->checkedOutTo->locale)

[what-the-diff]

PR Summary

• Updated conditions for "Checked Out" and "Checked In" events
  The team has refined the conditions within the "onCheckedOut" and "onCheckedIn" methods. Now, these methods adequately verify if a 'locale' is already set, thereby enhancing the functionality and avoiding potential errors. This change ensures a more reliable behavior when items are checked in or out.