jextract/jni: Support enums that have 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.
In cases where no enum cases exist, I will remove sealed to ensure that no compilation errors occur.

[ktoso]

Thanks!

[sidepelican]

I have found another problem again

error: cannot find type 'Nested' in scope
544 |   assert(selfPointer != 0, "selfPointer memory address was null")
545 |   let selfBits$ = Int(Int64(fromJNI: selfPointer, in: env$))
546 |   guard let self$ = UnsafeMutablePointer<Nested>(bitPattern: selfBits$) else {
    |                                          `- error: cannot find type 'Nested' in scope
547 |     fatalError("self memory address was null in call to \(#function)!")
548 |   }

[ktoso]

Hah ouch

[ktoso]

You know what, let's accept your other PR, it's simple enough. I don't think we need to over think this that much, not emitting the sealed interface will be fine