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.

[ktoso]

Hmmm, it may be better to emit but drop the "sealed" perhaps? Issues like "why didn't it get emitted" are annoying for users

[sidepelican]

Looks good, having fewer differences might be preferable.
I'll reconsider the implementation in that direction.

[ktoso]

Thank you!

[sidepelican]

New PR: #577

[ktoso]

#577 faced some complications, so let's accept this one actually, thank you!