DRILL-789: Left outer join returns "null" values for columns from the right table

The problem is that the "lastSet" field is not set on the Nullable mutator when the vector is loaded off the wire. The fix here is to make sure this gets set.

At the same time, this wouldn't be a problem, except for the fact that we fill in empty values when setValueCount is called, and if "lastSet" is not correct, we end up clobbering the existing data. We shouldn't need to call setValueCount on a transferred vector, so I am making this change in project record batch.
DRILL-809: BitVector.allocateNewSafe fixes.