Missing docvector in cw12b13

[lintool]

Capturing discussion with @AileenLin -

For cw12b13, for whatever reason, even with -storeDocvector during indexing, there is at least one document that doesn't have a doc vector. This means that when we try to perform any PRF, we might pull up a doc that doesn't have a doc vector. This happens with axiomatic. At this point, we don't know if we should throw an exception or just continue... because there's no way to tell if all docs are missing doc vectors (in which case we should throw an exception).

@AileenLin will trace upstream and try and figure out why doc vector in this case is null.