jextract/jni: Skip discriminator generation if enum has no cases

[sidepelican]

Currently, when the Swift enum has no cases, the generated Java code results in a compilation error.

error: sealed class must have subclasses
  public sealed interface Case {}
                ^

In Swift, it is a common idiom to use enums with no cases as namespaces.

I have updated the generator to skip the generation of any case-related code when an enum contains no cases.