[SPARK-5562] [MLLIB] LDA should handle empty document.

See the jira https://issues.apache.org/jira/browse/SPARK-5562

Author: Alok  Singh <singhal@Aloks-MacBook-Pro.local>
Author: Alok  Singh <singhal@aloks-mbp.usca.ibm.com>
Author: Alok Singh <“singhal@us.ibm.com”>

Closes #7064 from aloknsingh/aloknsingh_SPARK-5562 and squashes the following commits:

259a0a7 [Alok Singh] change as per the comments by @jkbradley
be48491 [Alok  Singh] [SPARK-5562][MLlib] re-order import in alphabhetical order
c01311b [Alok  Singh] [SPARK-5562][MLlib] fix the newline typo
b271c8a [Alok  Singh] [SPARK-5562][Mllib] As per github discussion with jkbradley. We would like to simply things.
7c06251 [Alok  Singh] [SPARK-5562][MLlib] modified the JavaLDASuite for test passing
c710cb6 [Alok  Singh] fix the scala code style to have space after :
2572a08 [Alok  Singh] [SPARK-5562][MLlib] change the import xyz._ to the import xyz.{c1, c2} ..
ab55fbf [Alok  Singh] [SPARK-5562][MLlib] Change as per Sean Owen's comments https://github.com/apache/spark/pull/7064/files#diff-9236d23975e6f5a5608ffc81dfd79146
9f4f9ea [Alok  Singh] [SPARK-5562][MLlib] LDA should handle empty document.