Make ConfigDataLocation.of non-nullable

[mhalbritter]

org.springframework.boot.context.config.ConfigDataLocation#of should never return null. Instead of the return null, we should throw an exception.

[mhalbritter]

If we throw an exception, we break #26342.

This branch explores returning a null object: https://github.com/mhalbritter/spring-boot/tree/mh/make-configdatalocation-of-non-nullable