[SPARK-9225] [MLLIB] LDASuite needs unit tests for empty documents

Add unit tests for running LDA with empty documents.
Both EMLDAOptimizer and OnlineLDAOptimizer are tested.

feynmanliang

Author: Meihua Wu <meihuawu@umich.edu>

Closes #7620 from rotationsymmetry/SPARK-9225 and squashes the following commits:

3ed7c88 [Meihua Wu] Incorporate reviewer's further comments
f9432e8 [Meihua Wu] Incorporate reviewer's comments
8e1b9ec [Meihua Wu] Merge remote-tracking branch 'upstream/master' into SPARK-9225
ad55665 [Meihua Wu] Add unit tests for running LDA with empty documents