As of Spring Data Lovelace SR4 / Spring Boot 2.1.2, a CrudRepository.findByIdOrNull(id: ID): T? = findById(id).orElse(null) Kotlin extension now provides out of the box a way to retrieve nullable entities in Spring Data.
If for performance reasons you would like to avoid the usage of Optional<T> wrapper, be aware that you have also the possibility to create a custom interface with a findFooById(id: ID): T? function. Query execution is store specific, but and most are using internally nullable values and will avoid the cost of Optional<T> wrapper. Notice this overhead should be negligible for most use cases, so using the builtin extension is recommended method.
See DATACMNS-1346 for more details.