Return empty set in BundleData#getTargetLanguages()

[yumaoka]

For now, BundleData#getTargetLanguages() may return empty set or null when there are no target languages, depending on how bundle is created (without target languages vs. empty target languages).

The distinction between null and empty set does not make sense for this case. The method should probably return empty set always if no target languages are available.

See also IBM-Cloud/gp-java-tools#43.