With JSpecify now a dependency of pkl-config-java, this moves the
non-null annotation to jspecify's.
This makes it simpler for users to do nullness checks, as tooling
already understands JSpecify nullness annotations.
This doesn't really make sense as part of the `Config` API.
We can maybe make class `ConfigUtils` public, but, I don't know how useful it
is anyways; it's more of an implementation detail.