MAHOUT-1232: VectorHelper.topEntries() throws a NPE when number of NonZero elements in vector < maxEntries - Adding a unit test to address the scenario when the input vector has all zero elements.

git-svn-id: https://svn.apache.org/repos/asf/mahout/trunk@1487645 13f79535-47bb-0310-9956-ffa450edef68