Major Description
Kuromoji user dictionary takes Reader and allows for comments and other lines to be ignored. But if its "empty" in the sense of no actual entries, the returned FST will be null, and it will throw a confusing NPE.
JapaneseTokenizer and JapaneseAnalyzer apis already treat null UserDictionary as having none at all, so I think the best fix is to fix the UserDictionary api from UserDictionary(Reader) to UserDictionary.open(Reader) or similar, and return null if the FST is empty.