[SPARK-4130][MLlib] Fixing libSVM parser bug with extra whitespace

This simple patch filters out extra whitespace entries.

Author: Joseph E. Gonzalez <joseph.e.gonzalez@gmail.com>
Author: Joey <joseph.e.gonzalez@gmail.com>

Closes #2996 from jegonzal/loadLibSVM and squashes the following commits:

e0227ab [Joey] improving readability
e028e84 [Joseph E. Gonzalez] fixing whitespace bug in loadLibSVMFile when parsing libSVM files