static Score::GetZero and Score::GetOne not working with many score types

[kho]

Currently both functions call ScoreCCandidate in SentenceScorer to get a ScoreP smart pointer (which, when non-null, gives access to non-static versions of Score::GetZero or Score::GetOne). However, most actual implementations of SentenceScorer simply return a null ScoreP pointer in ScoreCCandidate.