java_gen.PrimitiveSinkUtils: add support for putting nullable Strings, better support for null

Based on a conversation with @rlane, prepend optional entries with a
presence bit + length instead of the null-byte delimitation.
Merge into master from pull request #278:
java_gen.PrimitiveSinkUtils: add support for putting nullable Strings, better support for null (https://github.com/floodlight/loxigen/pull/278)