jextract/jni: Skip generating empty JNI cache enums

[sidepelican]

This is a small improvement to jextract.

Currently, a _JNI_ cache enum is generated for every class, even when it is not needed:

enum _JNI_MyClass {
} // printJNICache(_:_:) @ JExtractSwiftLib/JNISwift2JavaGenerator+SwiftThunkPrinting.swift:181

This enum is only utilized for enums with associated values.
For all other classes, it remains empty.
This PR updates the generator to skip generating these enums when they are empty.

[ktoso]

Thank you !