Better Handling of No LDA

[doofuslarge]

I know the whole purpose of lucene-lda is to run Lucene with LDA. However, to make the tool more general and useful, we need to gracefully accept cases when LDA is not desired, and instead only VSM indices need to be built and queried. This basic functionality works now, but we need to make sure we gracefully exit if the LDAHelper is empty.