jextract warnings: jni mode should better handle void result calls to avoid warnings

[ktoso]

jextract JNI source generation should handle void calls better, since we're getting warnings:

/Users/ktoso/code/swift-java/Samples/SwiftJavaExtractJNISampleApp/.build/plugins/outputs/swiftjavaextractjnisampleapp/MySwiftLibrary/destination/JExtractSwiftPlugin/Sources/MySwiftLibraryModule+SwiftJava.swift:472:13: warning: constant 'swiftResult$' inferred to have type '()', which may be unexpected
470 |       } // printTaskBody(printer:) @ JExtractSwiftLib/JNISwift2JavaGenerator+NativeTranslation.swift:976
471 |       do {
472 |         let swiftResult$ = await try MySwiftLibrary.asyncThrows()
    |             |- warning: constant 'swiftResult$' inferred to have type '()', which may be unexpected
    |             `- note: add an explicit type annotation to silence this warning