jextract: fail on missing or empty javaPackage (#919)

[bhanage-viraj]

Summary

• jextract no longer treats a missing/empty javaPackage as success and emit package ;. SwiftToJava.run() now requires a non-empty package before analysis or any generated output, for both JNI and FFM.
• --java-package stays optional so the value can still come from swift-java.config / the build plugin. Surrounding whitespace is rejected rather than trimmed; empty segments (com..foo) are rejected. The unnamed/default package is not implemented.
• Config docs now state that jextract requires a non-empty Java package name.
  Fixes jextract accepts an empty javaPackage and emits invalid Java source in JNI and FFM modes #919

Test plan

• swift test --filter JavaPackageValidationTests (nil, empty, whitespace, empty segments, valid package unchanged; SwiftToJava.run() for nil and "")
• swift test --filter InputSwiftTests (valid javaPackage still generates)
• CLI: --java-package '', omitted --java-package, and --mode ffm all fail with the missing-package error and write no output
• CLI: --java-package com.example.myswiftlibrary still succeeds and emits package com.example.myswiftlibrary;

[bhanage-viraj]

@ktoso This pull request is ready for review

[ktoso]

Thanks, some national holidays over here the next days, will review as soon as I can