SENSEI-326 Fix NPE in ScoreAugmentQuery (when inner scorer is null)

This appears to be a new bug in Sensei (since 1.6).
In Lucene 4, the scorer method on Weight may return null if the scorer will not match any docs. Thus, a new AugmentScorer should never be created if the inner scorer is null.

https://senseidb.atlassian.net/browse/SENSEI-326